error appears when the control is placed inside a RadTabItem

Scheduled for:
The fix for this issue will be available with LIB (version 2019.1.204) scheduled for publishing on Monday, 4th February 2019.

When you have the TabStripPlacement property set to something other than Top and you try to hide the TabControl border by setting the BorderThickness property to 0. The border is still shown. If you have TabStripPlacement set to Top the border disappears.

As a workaround set the BorderBrush property of the RadTabControl to "Transparent".

Scrolling in TabStrip does not work well when TabStripPlacement=Left or Bottom and  ScrollMode=Item. Pressing the Scroll Buttons scrolls the entire set of tabs.

Exceptions are occuring on ALL bindings, not just with RelativeSource

Command="{Binding AddCommand}"

will also raise an exception.

The workaround is to set the IsContentPreserved of the RadTabControl to True.

The TabItem doesn't changes when selected Item in the VS2010 designer.

UPDATE:
The issue is present also in the VS2008 and Blend.

When the SelectedIndex property is read during animation the CLR wrapper will throw InvalidCastException because of invalid cast to int. The object returned from GetValue is DeferredReference object.

In version 2016.2.613.45 of the Office2013 Theme, there is a problem in the control template for the RadTabControl. The ContentBorder element properly gets it's BorderThickness from the TemplatedParent, but the PlacementStates VisualStates override it with hard-coded Thickness (0,1,1,1 in the case of the "Left" Visual State) in their storyboards rather than doing an appropriate transform of the Thickness. This makes it impossible to override the BorderThickness for the control in any state other than the default layout without copying and replacing the entire control template. This appears to be a change in behavior since the (Februray 2016?) release, but I cannot be sure as the old theme files were uninstalled during the upgrade and the telerik.windows.controls.navigation.baml file is not properly decompiled in JustDecompile.

Workaround: Set HorizontalScrollBarVisibility property to Visible in order to show the scrollbar: 
<telerik:RadTabControl ScrollViewer.HorizontalScrollBarVisibility="Visible" .../>

or use Loaded event of TabControl and change the Height in order to trigger SizeChanged:
private void xTab_Loaded(object sender, RoutedEventArgs e)
{
    this.Height++;
    this.Height--;
}

The SelectedTabContent is not added when the control is loaded in memory and a we try to create a ImageSource out of the UserControl hosting the TabControl

When you have implicit DataTemplates applied to items the binding expression is lost on selection changed.

Selecting tabs with same names from the drop down menu always selects the first one.

In the VS2010 designer the selected tab is the last one even if the SelectedIndex proeprty of the TabControl is set to 0. The issue can be reprodiced in a scenario where the RadTabControl is defined in a RadWindow control.

Click event of a Button in the HeaderTemplate is not fired properly when another button outside the tab has IsDefault property set to true

SelectedItem does not respect the Reset collection changed action.

When the RadTabControl AllowDragReorder property is set to True and the control contains multiple rows with tabs, the dragging operation cannot be properly executed and completed.

When a TabItem has a width (and the TabControl - don't) and if you try to resize the window so that the width of the window is less than the width of the TabItem - the TabItems start jumping

When a RadDocking is located inside a RadTabControl and the RadPaneGroup contains only one RadPane the content is scrolled(if located inside a ScrollViewer)