When the window of an application that has a TabControl with Left/Right TabStripPlacement is resized, the measure of the TabStrip is incorrect and the ScrollButtons are in Disabled state. All Tabs should be visible when the window is in its normal state and should be scrollable.

A design time error occurs when referencing NoXaml assemblies. 

When pressing the Alt key, the access key of a given element must be displayed. Inside the Header of a TabItem it is not. The issue is not reproducible with the standard MS TabItem. It seems that it is related to the Foreground binding of the control. There is a workaround as shown below.

<telerik:RadTabControl>
    <telerik:RadTabItem >
        <telerik:RadTabItem.Header>
            
                <AccessText Text="_Tab 1"
                            Foreground="{Binding RelativeSource={RelativeSource AncestorType=telerik:RadTabItem},
                            Path=Foreground}"/>
        </telerik:RadTabItem.Header>
    </telerik:RadTabItem>
</telerik:RadTabControl>

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".

RadTabControl is currently designed to support scrolling of RadTabItems only when any of them is currently focused. An ability to scroll RadTabItems without explicitly focusing them should be provided.

RadTabControlBase expose a protected method called OnPreviewSelectionChanged that should raise the event PreviewSelectionChanged event. This means that you should be able to override it and implement additional logic if necessary.

However, the method is never used in the code and therefore it is not fired when the PreviewSelectionChanged event raises. 

Use the method to raise the event.

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.

By default, when selecting through the tab items, the first focusable element within each RadTabItem will gain focus. This is the behavior of the standard  RadTabItem as well. Provide an option to disable this default selection through a boolean property.

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.