Workaround:

public Form1()
{
    InitializeComponent();
     
    this.radRibbonBar1.KeyTipShowing+=radRibbonBar1_KeyTipShowing;
}

private void radRibbonBar1_KeyTipShowing(object sender, CancelEventArgs e)
{
    RadButtonElement b = sender as RadButtonElement;
    if (b!=null && b.Enabled==false)
    {
        e.Cancel = true;
    }
}

To reproduce:
Add RadRibbonForm and change the theme(for example to WIndows8Theme). Run the project. The system buttons are covered by the fill of RadRibbonBarElement.

Workaround:
Set the DrawFill property of RadRibbonBarElement to false.
this.RibbonBar.RibbonBarElement.DrawFill = false;

To reproduce:
- Set the theme and show the help button.

Workaround:
Hide the fill and the border in Visual Style builder

To reproduce:
1.Create a new Visual C# -> Telerik Windows Forms Application
2.Drag a RadRibbonBar control onto RadForm1
3.Create a single tab "First Tab" (otherwise you cannot collapse the ribbon)
4.Drag a button onto the form and add the following code to the click event handler:
RibbonTab tab = new RibbonTab("Test");
this.radRibbonBar1.CommandTabs.Add(tab);
tab.IsSelected = true;
5.You should end up with something like the attached png file.
6.Run the project and click the button. You should see the new tab is created and selected.
7.Close the project and run it again. This time, collapse the ribbon before clicking the button. No new tab is visible on the ribbon. Even if you click on the [First Tab] to show the full ribbon.
8.Click the expand ribbon button on the right and the new tab will then appear.

Workaround:
bool isExpanded = this.radRibbonBar1.Expanded;
this.radRibbonBar1.Expanded = true;
RibbonTab tab = new RibbonTab("Test " + this.radRibbonBar1.CommandTabs.Count); 
this.radRibbonBar1.CommandTabs.Add(tab);
tab.IsSelected = true;
this.radRibbonBar1.Expanded = isExpanded;

To reproduce:

1. Add a RadRibbonBar with a RadDropDownListElement and some other elements.
2. Use the following code:

for (int i = 0; i < 5; i++)
{
    RadListDataItem item = new RadListDataItem();
    item.Text = "Item" + i;
    this.radDropDownListElement1.Items.Add(item);
}
this.radRibbonBar1.EnableKeyMap = true;
this.radDropDownListElement1.KeyTip = "S";         
this.radDropDownListElement1.DropDownStyle = Telerik.WinControls.RadDropDownStyle.DropDownList; 
3. Press Alt, then 2. You will see the key tips for the drop down and its sibling items. 
4. Press S. You are expected to be navigated to the RadDropDownListElement, but are not. 

Here it is illustrated the behavior: http://screencast.com/t/Qmvqufznjt

Please refer to the attached gif file.

Workaround: close manually the popup by calling the this.radSplitButtonElement1.DropDownMenu.ClosePopup(Telerik.WinControls.UI.RadPopupCloseReason.Mouse) method.

To reproduce:
public RadRibbonForm1()
{
    InitializeComponent();
    radRibbonBar1.Expanded = false;
}     

private void button1_Click(object sender, EventArgs e)
{
    if (ribbonTab2.Visibility == Telerik.WinControls.ElementVisibility.Visible)
    {
        ribbonTab2.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
    }
    else
    {
        ribbonTab2.Visibility = Telerik.WinControls.ElementVisibility.Visible;
    }
}

Workaround:
private void button1_Click(object sender, EventArgs e)
{
  
    this.radRibbonBar1.RootElement.SuspendLayout();
    if (ribbonTab2.Visibility == Telerik.WinControls.ElementVisibility.Visible)
    {
        ribbonTab2.Visibility = Telerik.WinControls.ElementVisibility.Collapsed;
    }
    else
    {
        ribbonTab2.Visibility = Telerik.WinControls.ElementVisibility.Visible;
    }
    this.radRibbonBar1.RootElement.ResumeLayout(false);          
}

To reproduce: 
1. Add a RadRibbonBar and a RadButtonElement in one of the groups.
2. Set the RadButtonElement.UseCompatibleTextRendering property is set to false.
3. Set the RadButtonElement.Enabled property to false.

You will notice that the text is hardly read compared to version Q2 2014.

Workaround: Currently, the possible solution is to set the RadButtonElement.UseDefaultDisabledPaint property to false and subscribe to the EnabledChanged event where you can modify the ForeColor in order to make it gray when the button is disabled and apply a gray scaled image. 

To rperoduce:
- Set the themes as follows:
var theme = new Office2010BlackTheme();
new Telerik.WinControls.RadControlSpy.RadControlSpyForm().Show();
ThemeResolutionService.ApplicationThemeName = theme.ThemeName;

this.radRibbonBar1.ElementTree.EnableApplicationThemeName = false;
this.radRibbonBar1.ThemeName = "ControlDefault";

- Start and collapse the ribbon tabs. 
- Click on a tab to show its controls.

Workaround: Use the ApplyThemeToControlTree method instead of settig the ApplicationThemeName property.
ThemeResolutionService.ApplyThemeToControlTree(this, "Windows");

When you set the ApplicationMenuStyle property to  BackstageView at design time and set its  theme to Office2013Dark or Office2013Light you lose the ability to return back to the designer page  by clicking the back button.

Workaround: select  the RadRibbonBar via Visual Studio's Document Outline window (ctrl+W, U) or via Visual Studio's Property Grid's Component combobox (on the top of the property grid)

Please refer to the attached gif file.

Workaround: adjust manually the RadRibbonBarGroup.Margin when the expanded state is changed:

public Form1()
{
    InitializeComponent();
    this.radRibbonBar1.ExpandedStateChanged += radRibbonBar1_ExpandedStateChanged;
    radRibbonBar1.Expanded = false;
}

private void radRibbonBar1_ExpandedStateChanged(object sender, EventArgs e)
{
    if (this.radRibbonBar1.Expanded)
    {
        AdjustMargin(0);
    }
    else
    {
        AdjustMargin(-10);
    }
}

private void AdjustMargin(int top)
{
    foreach (RibbonTab tab in this.radRibbonBar1.CommandTabs)
    {
        foreach (RadRibbonBarGroup group in tab.Items)
        {
            group.Margin = new Padding(0, top, 0, 0);
        }
    }
}

To reproduce:
- Press and hold right mouse button.
- Move mouse cursor outside bounds of the button.
- Release mouse button - the button fill is not correct.

To reproduce:

public Form1()
{
    InitializeComponent();
    this.AllowAero = false;
    this.RibbonBar.Expanded = false;
    ThemeResolutionService.ApplicationThemeName = "TelerikMetro"; 
}

To reproduce:
1. Add a RadRibbonBar
2. Apply Office2013Light theme
3. Use the following code snippet:

public Form1()
{
    InitializeComponent();

    this.radRibbonBar1.CloseButton = false;
    this.radRibbonBar1.MinimizeButton = false;
    this.radRibbonBar1.MaximizeButton = false;
    this.radRibbonBar1.ApplicationMenuStyle = Telerik.WinControls.UI.ApplicationMenuStyle.ApplicationMenu;
}


Workaround:
this.radRibbonBar1.RibbonBarElement.ApplicationButtonElement.Margin = new Padding(3, 8, 0, 0);

To reproduce:
1. Add a RadRibbonBar and add several buttons with images as follows:

public Form1()
{
    InitializeComponent();

    RibbonTab tabItem1 = new RibbonTab();
    tabItem1.Text = "Manage";
    radRibbonBar1.CommandTabs.Add(tabItem1);

    RadRibbonBarGroup radRibbonBarGroup1 = new RadRibbonBarGroup();
    radRibbonBarGroup1.Text = "radRibbonBarGroup1";
    tabItem1.Items.Add(radRibbonBarGroup1);

    RadButtonElement radButtonElement1 = new RadButtonElement();
    radButtonElement1.Image = Properties.Resources.about;
    radButtonElement1.TextImageRelation = TextImageRelation.ImageBeforeText;
    radButtonElement1.Text = "First Button";
    radRibbonBarGroup1.Items.Add(radButtonElement1);

    RadButtonElement radButtonElement2 = new RadButtonElement();
    radButtonElement2.Image = Properties.Resources.about;
    radButtonElement2.TextImageRelation = TextImageRelation.ImageBeforeText;
    radButtonElement2.Text = "Second Button";
    radRibbonBarGroup1.Items.AddRange(new RadItem[] { radButtonElement1, radButtonElement2 });

    RadRibbonBarButtonGroup radRibbonBarButtonGroup1 = new RadRibbonBarButtonGroup();
    radRibbonBarButtonGroup1.Orientation = System.Windows.Forms.Orientation.Vertical;
    radRibbonBarButtonGroup1.ShowBorder = true;

    RadButtonElement radButtonElement3 = new RadButtonElement();
    radButtonElement3.Text = "Button One";
    radButtonElement3.Image = Properties.Resources.about;
    radButtonElement3.TextImageRelation = TextImageRelation.ImageBeforeText;

    RadButtonElement radButtonElement4 = new RadButtonElement();
    radButtonElement4.Text = "Button Two";
    radButtonElement4.Image = Properties.Resources.about;
    radButtonElement4.TextImageRelation = TextImageRelation.ImageBeforeText;
  
    radRibbonBarButtonGroup1.Items.AddRange(new RadItem[] { radButtonElement3, radButtonElement4 });
    radRibbonBarGroup1.Items.Add(radRibbonBarButtonGroup1);
}

2. Run the applications, re-size the form horizontally until all buttons are collapsed to smallest size, then click the form's maximize button. As a result all buttons are drawn without text.

Workaround:
protected override void WndProc(ref Message message)
{
    const int WM_SYSCOMMAND = 0x0112;
    const int SC_MAXIMIZE = 0xF030; 

    switch (message.Msg)
    {
        case WM_SYSCOMMAND:
            int command = message.WParam.ToInt32() & 0xfff0;
            if (command == SC_MAXIMIZE) 
            {
                this.Size = new System.Drawing.Size(300, 300);
            }
            break;
    }
    base.WndProc(ref message);
}

To reproduce:
- Add a ribbon bar to a blank form and set the theme to Office2013Light
- Disable the title bar buttons:
this.radRibbonBar1.MaximizeButton = false;
this.radRibbonBar1.MinimizeButton = false;
this.radRibbonBar1.CloseButton = false;

- The buttons are still visible when the backstage view button is clicked.

Workaround:
radRibbonBar1.BackstageControl.BackstageElement.TitleBarElement.CloseButton.Visibility = Telerik.WinControls.ElementVisibility.Hidden;
radRibbonBar1.BackstageControl.BackstageElement.TitleBarElement.MaximizeButton.Visibility = Telerik.WinControls.ElementVisibility.Hidden;
radRibbonBar1.BackstageControl.BackstageElement.TitleBarElement.MinimizeButton.Visibility = Telerik.WinControls.ElementVisibility.Hidden;

To reproduce:

Add a RadRibbonBar, a tab and a group. In the group add a RadButtonElement. Add two buttons on the form. One for enabling and one for disabling the RadButtonElement.  Start the application , collapse the RadRibbon and disable the button inside. Now expand it and you will see the visual bug.

Workaround:

Subscribe to the ExpandedChanged event of the RadRibbonBar and if the RadRibbonBar is expanded and the button is disabled re-enable and re-disable the button. This will correct its visual appearance. 

To reproduce:
- Add a tab to a RadRibbonForm form.
- Collapse the ribbon and press the Alt key to show the KeyTips.
- Select a tab by pressing the corresponding key.
- You will notice that the tab is not expanded.

Workaround:
void radRibbonBar1_KeyTipActivating(object sender, CancelEventArgs e)
{
    keyExpanded = true;
}

void radRibbonBar1_Click(object sender, EventArgs e)
{
    keyExpanded = false;
}

bool keyExpanded = false;

void radRibbonBar1_CommandTabSelected(object sender, Telerik.WinControls.UI.CommandTabEventArgs args)
{
    if (!radRibbonBar1.Expanded && keyExpanded)
    {
        radRibbonBar1.Expanded = true;
    }
}

To reproduce: 
1.Add a RadForm.
2.Create a UserControl with RadRibbonBar.
3.Apply Window8 theme to the RadRibbonBar and to the RadForm.
4.Use the following code:

Me.RadRibbonBar1.CloseButton = False
Me.RadRibbonBar1.MinimizeButton = False
Me.RadRibbonBar1.MaximizeButton = False
Me.RadRibbonBar1.RibbonBarElement.Text = String.Empty

5.When you add the UserControl to the RadForm and run the application, the QuickAccessToolbar items are cut off.

Workaround:

Me.RadRibbonBar1.RibbonBarElement.QuickAccessToolBar.OverflowButtonElement.Padding = New Padding(0, 0, 0, -15)
Me.RadRibbonBar1.RibbonBarElement.QuickAccessToolBar.InnerItem.Padding = New Padding(0, 10, 0, -10)