Steps to reproduce.

1. Add a RadScheduler to a form, add some appointments.
2. Add grouping with at least two groups.
3. Drag an appointment from the second group (top to bottom) and drop it above the scheduler. The drag hint should be for invalid operation (striked red circle)
You will see that the appointment will disappear from the scheduler.

WORKAROUND
AddHandler Me.RadScheduler1.DragDropBehavior.Stopped, AddressOf DragDropBehavior_Stopped

Private Sub DragDropBehavior_Stopped(sender As Object, e As EventArgs)
    Dim element As TimelineGroupingByResourcesElement = Me.RadScheduler1.SchedulerElement.Children(0)

    For Each timelineViewElement In element.Children
        If timelineViewElement.GetType() = GetType(SchedulerTimelineViewElement) Then
            For Each applicationElement As RadElement In CType(timelineViewElement, SchedulerTimelineViewElement).Presenter.Children
                If applicationElement.GetType() = GetType(AppointmentElement) Then
                    applicationElement.Visibility = ElementVisibility.Visible
                End If
            Next
        End If
    Next
End Sub

RadScheduler printing does not take in consideration the Ruler scale and always print with 30 minutes timescale.

ScrollToWorkHours does not work properly in DayViewGroupedByResource mode

To work around the issue, you can set scheduler's GroupType to None, set the  ResourcesPerView property, set the GroupType to Resource:

radScheduler1.GroupType = GroupType.None;
this.radScheduler1.ActiveView.ResourcesPerView =resourcesPerView 
radScheduler1.GroupType = GroupType.Resource;

When in multi day view if many days are being added, each day visual becomes so narrow that the application practically becomes useless. If there is horizontal scroll bar the view could be stretched and the user would be able to scroll between the days.

If there is WeekDays option set in monthly recurrence rule and try to get the occurence start the method just adds the interval to the start day, rather than considering the WeekDays option.

CLOSED: Not an issue. The method is not intended to return real occurrence dates but rather dates that are potential occurrences and must be checked. The method is not correctly named but renaming it will result in a breaking change. The description of the method is updated. To achieve the desired functionality use OccurrenceEnumerator.

this.radScheduler1.Appointments.EndUpdate();

Does not refresh the Scheduler view element.

IMPORVE. RadScheduler - improve the selection of the cells into RadScheduler to support Ctrl-key.
This will allow to users to select cells/days with space between them.

CLOSED: The selection in scheduling components should be a single range of two dates in which users can create a single appointment after selected. There would be no purpose in multiple range selection as there are no actions the users can perform with them after selecting.

IMPROVE. RadScheduler - Add CellSelected event that will be firing when the cell is selected.

RadScheduler -  e.Appointment.BackgroundId property in AppointmentFormatting event does not work correctly, when you open Edit Appointment  dialog form to edit the Background color of selected appointment.

Steps to reproduce:
1. Subscribe to AppointmentFormatting event and set e.Appointment.BackgroundId property.
2. Create new Appointment via Edit Appointment dialog form.
3. Open the the appointment via Edit Appointment dialog form and change the Background color.
4. Close the Edit Appointment dialog form.

Work around:
Do not set this property in AppointmentFormatting event.

CLOSED: Modifying data properties of the appointment should not be done on the Formatting events. The Formatting events stand for modifying visual properties of the elements being rendered and are fired during element creation. Modifying any data property of the active view or of the appointments will call another element creation cycle recursively and might result in StackOverflow exceptions or ElementAlreadyAdded exceptions.

Steps to reproduce:
            radScheduler.ActiveViewType = SchedulerViewType.Timeline;
            radScheduler.GetTimelineView().ShowTimescale(Timescales.Hours);
            radScheduler.GetTimelineView().GetTimescale(Timescales.Hours).DisplayedCellsCount = 10;//set 8 for workarround.

            radScheduler.ActiveView.StartDate = DateTime.Today;
            radScheduler.GetTimelineView().RangeStartDate = DateTime.Today.AddHours(7);
            radScheduler.GetTimelineView().RangeEndDate = DateTime.Today.AddDays(1);
            radScheduler.GetTimelineView().ResourcesPerView = 5;

            radScheduler.GroupType = GroupType.Resource;
            radScheduler.SchedulerElement.SetResourceHeaderAngleTransform(SchedulerViewType.Timeline, 0);

Check out the Outlook vertical scrolling abilities in MonthView

Let's say that you have an appoiment that starts at 8:30 and ends at 8:30. RadScheduler will display it as it should, but it will not be considered by the printing functionality.
We should also have in mind the case where there are several appointments starting from 8:30 and ending at 8:30.

As an example, if an appointment starts at 12 o'clock on one day and ends at 12 o'clock on the next day, it should be positioned to start in the middle of the first day cell and end in the middle of the next day, instead of filling both days.

To reproduce:
RadSchedulerLocalizationProvider.CurrentProvider = new CustomSchedulerLocalizationProvider();

radScheduler1.Appointments.Add(new Appointment(DateTime.Now.AddDays(-5), DateTime.Now.AddDays(-4)));
radScheduler1.Appointments.Add(new Appointment(DateTime.Now.AddDays(5), DateTime.Now.AddDays(4)));

Workaround:
radScheduler1.SchedulerElement.Refresh();

The width of a resource does not always match the column(s) underneath it, resulting in the resources shifting to the left in relation to the column(s) below. It appears to be about 1 pixel  difference per resource which becomes more noticeable with multiple resources .If you resize the form the resources will come close to matching the column(s) before jumping back to the full difference again. This issue can be demonstrated in the Scheduler->Grouping demo

Currently, a time range of working ours is valid for all days in the week. There could be a rule that sets a specific range for some days of the week, leaving another range of hours for the rest of the days.

ADD. Scheduler - add possibility to increase the height of the line showing the current time

Steps to reproduce:
1. Add a RadSchduler to a form (No need to add appointments) 
2. Set the active view type to Timeline 
3. Set the Appointmentheight property of the timeline view element to 50 
4. Run the project, right-click on the scheduler and select Next View You will see a NullReferenceException exception.

The issue is when you drag up or drag down the top or bottom of the appointment i.e extend or decrease the slot.
The error I get with that same codebase is
Object reference not set to an instance of an object.
inner exception null

Source: Telerik.WinControls.Scheduler

StackTrace:
at Telerik.WinControls.UI.DayViewAppointmentsTable.appointment_MouseUp(Object sender, MouseEventArgs e)
   at Telerik.WinControls.UI.DayViewAppointmentsTable.AppointmentMouseUp(Object sender, MouseEventArgs e)
   at Telerik.WinControls.UI.RadSchedulerElement.scheduler_AppointmentElementMouseUp(Object sender, MouseEventArgs e)
   at Telerik.WinControls.UI.RadScheduler.OnAppointmentElementMouseUp(Object sender, MouseEventArgs args)...