How to reproduce: check the attached (video radscheduler-selection-incorrect.gif)
 public RadForm1()
 {
     InitializeComponent();

     Appointment appointment = new Appointment(DateTime.Today.AddHours(13), TimeSpan.FromHours(1), "Test Appointment");
     this.radScheduler1.Appointments.Add(appointment);

     for (int i = 0; i < 25; i++)
     {
         appointment = new Appointment(DateTime.Today.AddHours(24), TimeSpan.FromHours(1), "AllDay: " + i);
         appointment.AllDay = true;
         this.radScheduler1.Appointments.Add(appointment);
     }
     this.radScheduler1.AllowAppointmentsMultiSelect = true;
 }

Workaround: create a custom input behavior

this.radScheduler1.SchedulerInputBehavior = new CustomSchedulerInputBehavior(this.radScheduler1);

public class CustomSchedulerInputBehavior : SchedulerInputBehavior
{
    public CustomSchedulerInputBehavior(RadScheduler scheduler) 
        : base(scheduler)
    {
    }

    public override bool HandleMouseWheel(MouseEventArgs args)
    {
        if (!this.Scheduler.AllowMouseWheelSupport)
            return false;

        bool scrolled = false;

        if (this.Scheduler.SelectionBehavior.IsAllDayAreaSelection || this.IsLastSelectedAppointmentAllDay(this.Scheduler.SelectionBehavior.SelectedAppointments))
        {
            if (this.Scheduler.GroupType == GroupType.Resource)
            {
                SchedulerDayViewGroupedByResourceElement grouped = this.Scheduler.ViewElement as SchedulerDayViewGroupedByResourceElement;
                IList<SchedulerDayViewElement> childViews = grouped != null ? grouped.GetChildViewElements() : null;
                if (childViews != null && childViews.Count > 0)
                {
                    RadScrollBarElement scroll = childViews[childViews.Count - 1].AllDayHeaderElement.ScrollBar;
                    if (scroll.Visibility != ElementVisibility.Collapsed)
                    {
                        int newValue = scroll.Value - childViews[childViews.Count - 1].AllDayHeaderElement.HeaderHeight * Math.Sign(args.Delta);
                        newValue = Math.Max(Math.Min(newValue, scroll.Maximum - scroll.LargeChange + 1), scroll.Minimum);
                        scroll.Value = newValue;
                        scrolled = true;
                    }
                }
            }
            else
            {
                SchedulerDayViewElement dayView = this.Scheduler.ViewElement as SchedulerDayViewElement;
                RadScrollBarElement scroll = dayView != null ? dayView.AllDayHeaderElement.ScrollBar : null;

                if (scroll != null && scroll.Visibility != ElementVisibility.Collapsed)
                {
                    int newValue = scroll.Value - dayView.AllDayHeaderElement.HeaderHeight * Math.Sign(args.Delta);
                    newValue = Math.Max(Math.Min(newValue, scroll.Maximum - scroll.LargeChange + 1), scroll.Minimum);
                    scroll.Value = newValue;
                    scrolled = true;
                }
            }
        }

        if (scrolled)
        {
            return false;
        }

        if (args.Delta > 0)
        {
            this.Scheduler.ViewElement.Scroll(true);
        }
        else
        {
            this.Scheduler.ViewElement.Scroll(false);
        }
        return false;
    }

    private bool IsLastSelectedAppointmentAllDay(ReadOnlyCollection<IEvent> selectedAppointments)
    {
        if (selectedAppointments.Count > 0)
        {
            return selectedAppointments[selectedAppointments.Count - 1].AllDay;
        }

        return false;
    }

}

Workaround: create a custom Appointment and override the PaintRecurrenceIcon method

Public Class MyAppointmentElement
    Inherits AppointmentElement

    Public Sub New(scheduler As RadScheduler, view As SchedulerView, appointment As IEvent)
        MyBase.New(scheduler, view, appointment)
    End Sub

    Public Overrides Sub PaintRecurrenceIcon(graphics As IGraphics)

        If Not Me.Recurring Then
            Return
        End If

        Dim icon As Image = DirectCast(GetType(AppointmentElement).GetMethod("GetRecurrenceIcon", BindingFlags.Instance Or BindingFlags.NonPublic).Invoke(Me, Nothing), Image)

        If icon Is Nothing Then
            Return
        End If

        SyncLock icon
            Dim clientRect As RectangleF = Me.GetClientRectangle(Me.Bounds.Size)
            Dim x As Integer = CInt(clientRect.X) + CInt(clientRect.Width) - icon.Width

            If Me.RightToLeft Then
                x = CInt(clientRect.X)
            End If

            Dim imageRect As Rectangle = New Rectangle(x, CInt(clientRect.Y) + CInt(clientRect.Height) - icon.Height, icon.Width, icon.Height)
            graphics.DrawImage(imageRect, icon, ContentAlignment.TopLeft, True)
        End SyncLock
    End Sub
End Class

Public Class MyElementProvider
    Inherits SchedulerElementProvider
    Public Sub New(scheduler As RadScheduler)
        MyBase.New(scheduler)
    End Sub
    Protected Overrides Function CreateElement(Of T As SchedulerVisualElement)(view As SchedulerView, context As Object) As T
        If GetType(T) = GetType(AppointmentElement) Then
            Return TryCast(New MyAppointmentElement(Me.Scheduler, view, DirectCast(context, IEvent)), T)
        End If
        Return MyBase.CreateElement(Of T)(view, context)
    End Function
End Class

If you set the ResourcesPerView property first to a value greater than the available resources in RadScheduler and then add even more resources, incorrect layout is displayed. Please refer to the attached gif file. However, note that if you first add the resources and then manipulate the ResourcesPerView property, everything is OK. 

Workaround: set the ResourcesPerView property considering the available resources in RadScheduler

    Private Sub RadSpinEditor2_ValueChanged(sender As Object, e As EventArgs) Handles RadSpinEditor2.ValueChanged
        Me.RadScheduler1.Resources.Clear()

        For i As Integer = 1 To Me.RadSpinEditor2.Value
            Dim resource As New Telerik.WinControls.UI.Resource()
            resource.Id = New EventId(i)
            resource.Name = "Resource" & i
            resource.Visible = True
            resource.Color = Color.LightBlue

            Me.RadScheduler1.Resources.Add(resource)
        Next i
         
        Me.RadScheduler1.GroupType = GroupType.Resource
        Me.RadScheduler1.ActiveView.ResourcesPerView = Math.Min(Me.RadScheduler1.Resources.Count, Me.RadSpinEditor1.Value)
    End Sub

To reproduce:

            this.radScheduler1.ActiveViewType = Telerik.WinControls.UI.SchedulerViewType.Month;

            SchedulerMonthView monthView = this.radScheduler1.GetMonthView();
            DateTime start= new DateTime(2018, 8, 1);
            DateTime end= new DateTime(2018,8,31);
            monthView.ShowFullMonth = true;
            monthView.StartDate=start;
            monthView.RangeStartDate = start;
            monthView.RangeEndDate = end;

You will notice that you can navigate outside the specified range.

Workaround: manipulate the start date of the view:
this.radScheduler1.ActiveView.PropertyChanged += ActiveView_PropertyChanged;

        DateTime start = new DateTime(2018, 8, 1);
        DateTime end = new DateTime(2018,8,31);

        private void ActiveView_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            if (e.PropertyName == "StartDate" && (this.radScheduler1.ActiveView.StartDate < start || start.AddDays(30) < this.radScheduler1.ActiveView.StartDate) 
                || e.PropertyName == "WeekCount" && this.radScheduler1.ActiveView.EndDate > end)
            {
                this.radScheduler1.ActiveView.StartDate = start;
            }
        }

Use attached to reproduce.

Workaround:
protected override void OnShown(EventArgs e)
{
    base.OnShown(e);
    radScheduler1.SchedulerElement.ViewElement.UpdateCells();      
}

How to rerproduce:
this.radScheduler1.AccessibleInterval.Start = new DateTime(2018, 7, 18, 00, 00, 00).AddMonths(-2);
            this.radScheduler1.AccessibleInterval.End = new DateTime(2018, 7, 18, 00, 00, 00).AddMonths(2);
            this.radScheduler1.ActiveViewType = Telerik.WinControls.UI.SchedulerViewType.Month;

Workaround:
public class CustomRadScheduler : RadScheduler
{
    private FieldInfo schedulerFi;
    private FieldInfo activeViewsFi;
    private MethodInfo onPropertyChangedMi;
    private MethodInfo setActiveViewMi;

    protected override void CreateChildItems(RadElement parent)
    {
        base.CreateChildItems(parent);

        this.activeViewsFi = typeof(RadScheduler).GetField("activeViews", BindingFlags.Instance | BindingFlags.NonPublic);
        this.schedulerFi = typeof(SchedulerView).GetField("scheduler", BindingFlags.Instance | BindingFlags.NonPublic);
        this.onPropertyChangedMi = typeof(SchedulerView).GetMethod("OnPropertyChanged", BindingFlags.Instance | BindingFlags.NonPublic);
        this.setActiveViewMi = typeof(RadScheduler).GetMethod("SetActiveView", BindingFlags.Instance | BindingFlags.NonPublic);
    }

    public override string ThemeClassName
    {
        get
        {
            return typeof(RadScheduler).FullName;
        }
    }

    /// <summary>
    /// Gets or sets the type of the active view.
    /// </summary>
    /// <value>The type of the active view.</value>
    [DefaultValue(SchedulerViewType.Day)]
    [NotifyParentProperty(true)]
    public override SchedulerViewType ActiveViewType
    {
        get
        {
            return base.ActiveViewType;
        }
        set
        {
            if (this.ActiveViewType != value && value == SchedulerViewType.Month)
            {
                SchedulerView newView;
                Dictionary<SchedulerViewType, SchedulerView> activeViews = this.activeViewsFi.GetValue(this) as Dictionary<SchedulerViewType, SchedulerView>;
                if (activeViews.ContainsKey(value))
                {
                    newView = activeViews[value];
                }
                else
                {
                    SchedulerView view = new CustomSchedulerMonthView();
                    this.schedulerFi.SetValue(view, this);
                    this.onPropertyChangedMi.Invoke(view, new object[] { new string[] { "Scheduler" } });
                    newView = view;
                }

                if (this.ActiveView != newView && newView != null)
                {
                    this.setActiveViewMi.Invoke(this, new object[] { newView, true });
                }
            }
            else
            {
                base.ActiveViewType = value;
            }
        }
    }
}

public class CustomSchedulerMonthView : SchedulerMonthView
{
    public override SchedulerView OffsetView(int offset)
    {
        if (this.ShowFullMonth)
        {
            DateTime dtStart = DateHelper.GetStartOfMonth(this.StartDate);
            if (this.StartDate.Day > 1)
            {
                dtStart = dtStart.AddMonths(1);
            }

            dtStart = dtStart.AddMonths(offset);
            return this.CreateViewWithStartDate(dtStart);
        }
        else
        {
            DateTime startDate = this.StartDate.Add(new TimeSpan(offset * this.OffsetTimeSpan.Ticks));
            DateTimeInterval interval = new DateTimeInterval(startDate, this.GetEndDate(startDate));

            if (this.Scheduler.AccessibleInterval.Contains(interval))
            {
                return this.CreateViewWithStartDate(startDate);
            }

            if (true)
            {

            }

            return this.CreateViewWithStartDate(startDate);
        }
    }

    protected override SchedulerView CreateViewWithStartDate(DateTime startDate)
    {
        SchedulerMonthView monthView = new SchedulerMonthView();
        this.CopyPropertiesToView(monthView);
        DateTimeInterval interval = new DateTimeInterval(startDate, this.GetEndDate(startDate));
        if (interval.End > this.Scheduler.AccessibleInterval.End)
        {
            startDate = startDate.Add(new TimeSpan(-1 * this.OffsetTimeSpan.Ticks));
        }

        if (interval.Start < this.Scheduler.AccessibleInterval.Start)
        {
            startDate = startDate.Add(new TimeSpan(1 * this.OffsetTimeSpan.Ticks));
        }


        monthView.StartDate = startDate;

        if (this.ShowFullMonth)
        {
            monthView.WeekCount = DateHelper.GetMonthDisplayWeeks(startDate, this.CurrentCulture.DateTimeFormat);
        }

        return monthView;
    }
}

How to reproduce:
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();
        
        this.radScheduler1.MouseMove += RadScheduler1_MouseMove;
        this.radScheduler1.GroupType = GroupType.Resource;

        Appointment appointment = new Appointment(DateTime.Today.AddHours(13), TimeSpan.FromHours(1), "Test Appointment");
        this.radScheduler1.Appointments.Add(appointment);
    }

    private void RadScheduler1_MouseMove(object sender, MouseEventArgs e)
    {
        Point pt = this.radScheduler1.PointToClient(Cursor.Position);
        SchedulerCellElement cell = this.radScheduler1.SchedulerElement.ElementTree.GetElementAtPoint(pt) as SchedulerCellElement;

        if (cell != null)
        {
            if (cell.Date != null)
            {
                Console.WriteLine(cell.Date.ToShortTimeString());
            }
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        switch (this.radScheduler1.ActiveViewType)
        {
            // showing the Day View
            case SchedulerViewType.Day:

                var theDayView = this.radScheduler1.GetDayView();
                if (theDayView != null)
                {
                    RulerPrimitive ruler = (this.radScheduler1.SchedulerElement.ViewElement as SchedulerDayViewElement).DataAreaElement.Ruler;
                    ruler.RangeFactor = ScaleRange.QuarterHour;
                    ruler.StartScale = 8;
                    ruler.EndScale = 18;
                }
                break;
        }
    }
}

Workaround: instead of accessing directly the ruler, apply the scaling on the view element
private void button2_Click(object sender, EventArgs e)
{
    switch (this.radScheduler1.ActiveViewType)
    {
        // showing the Day View
        case SchedulerViewType.Day:

            var theDayView = this.radScheduler1.GetDayView();
            if (theDayView != null)
            {
                theDayView.RangeFactor = ScaleRange.QuarterHour;
                theDayView.RulerStartScale = 8;
                theDayView.RulerEndScale = 18;
            }
            break;
    }
}

By design, the month view in RadScheduler stacks the appointments for a certain day considering the start time of the appointments and their duration. Internally, it uses MonthViewEventsComparer which would be great to be replaceable somehow. 

Add the possibility to specify given date as a holiday.

It is possible to schedule multiple appointments in the same time period. In some scenario's this should be avoided. When the agenda can change from outside the scheduler, sometimes conflics will apear when the resource is not visible on the scheduler. It would be nice to have a list of all the conflicted appointments

To reproduce:
 public Form1()
 {
     InitializeComponent();

     for (int i = 0; i < 7; i++)
     {
         this.radScheduler1.Appointments.Add(new Appointment(DateTime.Now.AddHours(i),TimeSpan.FromHours(3),"App" + i));
     }
     this.radScheduler1.AutoSizeAppointments = true;
     this.radScheduler1.ActiveViewType = SchedulerViewType.Month;
     SchedulerMonthView monthView = this.radScheduler1.GetMonthView();
    
     monthView.EnableAppointmentsScrolling = true;
 }

Workaround: set the AutoSizeAppointments property to false.

How to reproduce:
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();

        Resource resource1 = new Resource();
        resource1.Id = new EventId(1);
        resource1.Name = "Resource 1";
        resource1.Color = Color.Blue;

        Resource resource2 = new Resource();
        resource2.Id = new EventId(2);
        resource2.Name = "Resource 2";
        resource2.Color = Color.Green;

        Resource resource3 = new Resource();
        resource3.Id = new EventId(3);
        resource3.Name = "Resource 3";
        resource3.Color = Color.Red;

        this.radScheduler1.Resources.Add(resource1);
        this.radScheduler1.Resources.Add(resource2);
        this.radScheduler1.Resources.Add(resource3);
    }

    private void RadForm1_Load(object sender, EventArgs e)
    {
        this.radScheduler1.GroupType = GroupType.Resource;
        ((SchedulerViewGroupedByResourceElementBase)this.radScheduler1.ViewElement).ResourceStartIndex = 2;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        this.radScheduler1.Resources.RemoveAt(0);
    }
}


Workaround: 
public partial class RadForm1 : Telerik.WinControls.UI.RadForm
{
    public RadForm1()
    {
        InitializeComponent();

        Resource resource1 = new Resource();
        resource1.Id = new EventId(1);
        resource1.Name = "Resource 1";
        resource1.Color = Color.Blue;

        Resource resource2 = new Resource();
        resource2.Id = new EventId(2);
        resource2.Name = "Resource 2";
        resource2.Color = Color.Green;

        Resource resource3 = new Resource();
        resource3.Id = new EventId(3);
        resource3.Name = "Resource 3";
        resource3.Color = Color.Red;

        this.radScheduler1.Resources.Add(resource1);
        this.radScheduler1.Resources.Add(resource2);
        this.radScheduler1.Resources.Add(resource3);
    }

    private void RadForm1_Load(object sender, EventArgs e)
    {
        this.radScheduler1.GroupType = GroupType.Resource;
        ((SchedulerViewGroupedByResourceElementBase)this.radScheduler1.ViewElement).ResourceStartIndex = 2;
    }

    private void radButton1_Click(object sender, EventArgs e)
    {
        this.radScheduler1.GroupType = GroupType.None;
        this.radScheduler1.Resources.RemoveAt(0);
        this.radScheduler1.GroupType = GroupType.Resource;
    }
}

Please refer to the attached gif file and sample project.

Workaround: remove the database restrictions and validate the data in the edit dialog before submitting the new appointment data.

To reproduce:
this.radScheduler1.RightToLeft = System.Windows.Forms.RightToLeft.Yes;
this.radScheduler1.Appointments.Add(new Appointment(DateTime.Now, TimeSpan.FromHours(2), "آقای حقیقت"));

Please refer to the attached screenshot.

Workaround:

private void radScheduler1_AppointmentFormatting(object sender, SchedulerAppointmentEventArgs e)
{ 
    e.AppointmentElement.UseHtml = false; 
}

To reproduce:
RadScheduler radScheduler1 = new RadScheduler();
public Form1()
{
    InitializeComponent();
    radScheduler1.Top += 100;
    radScheduler1.Parent = this;
    this.radSchedulerNavigator1.AssociatedScheduler = radScheduler1;
    this.radScheduler1.ActiveViewType = SchedulerViewType.Timeline;
  

    Color[] colors = new Color[]{Color.LightBlue, Color.LightGreen, Color.LightYellow,
        Color.Red, Color.Orange, Color.Pink, Color.Purple, Color.Peru, Color.PowderBlue};
    string[] names = new string[]{"Alan Smith", "Anne Dodsworth",
         "Boyan Mastoni", "Richard Duncan", "Maria Shnaider"};
    for (int i = 0; i < names.Length; i++)
    {
        Resource resource = new Resource();
        resource.Id = new EventId(i);
        resource.Name = names[i];
        resource.Color = colors[i];
   this.radScheduler1.Resources.Add(resource);
    }

    this.radScheduler1.GroupType = GroupType.Resource;
    this.radScheduler1.ActiveView.ResourcesPerView = 2;
}

private void radButton1_Click(object sender, EventArgs e)
{
    SchedulerWeeklyCalendarPrintStyle weeklyCalendarStyle = new SchedulerWeeklyCalendarPrintStyle();
    weeklyCalendarStyle.AppointmentFont = new System.Drawing.Font("Segoe UI", 12, FontStyle.Regular);
    weeklyCalendarStyle.HeadingAreaHeight = 120;
    weeklyCalendarStyle.HoursColumnWidth = 30;

    this.radScheduler1.PrintStyle = weeklyCalendarStyle;

    this.radScheduler1.PrintPreview();
}

Workaround:
Change the view before printing.

To reproduce:
- Use the search in the scheduler navigator.
- Sort the grid by start/end date.
- The values are sorted as strings.

Workraround:
- Use custom sorting.

To reproduce:

1. Add a RadScheduler and associate a RadSchedulerNavigator to it.
2. Change the culture to  "fa-IR". When running the application you will notice that the time range in the navigator is not displayed correctly considering the culture of the associate RadScheduler.

Note: you can set the SchedulerNavigatorElement.DateFormat property to  "dd dddd". Thus, the time range should be displayed in a similar way as the header cells in RadScheduler.

Workaround: you can controls what to be displayed in the navigator by the SchedulerNavigatorElement.DateLabel.Text property.

            this.radScheduler1.Culture = new System.Globalization.CultureInfo("fa-IR");

            string start = this.radScheduler1.ActiveView.StartDate.ToString(this.radSchedulerNavigator1.SchedulerNavigatorElement.DateFormat, this.radScheduler1.Culture);
            string end = this.radScheduler1.ActiveView.EndDate.ToString(this.radSchedulerNavigator1.SchedulerNavigatorElement.DateFormat, this.radScheduler1.Culture);
            this.radSchedulerNavigator1.SchedulerNavigatorElement.DateLabel.Text = start + " - " + end;

Currently RadScheduler can only group the appointments first by resource and then by date. It should be possible to group the appointments in day view and in week view first by date and then by resource.