Specifically this bug was found in WPF, I'm not sure of its state in Silverlight.
The FontWeight is permanently some shade of Bold when using Metro.
This is demonstrated in the Telerik WPF demos in the ComboBox/Theming demo.

After RadComboBoxs DropDown (PopUp) is open moving the focus with Shift+Tab skips the Combo, Tab does not skip it


Available in R2 2015 Release (v.2015.2.623.40)

Available in the R1 2017 SP1 Release.

Place a RadComboBox in the GridViewColumn header and set SelectedIndex in the xaml. The SelectionChanged event is fired, but the command is not executed.

As a workaround you can set the SelectedIndex in xaml instead of on the Loaded event,

The OnLostFocus is triggered when the control gets the focus and is not triggered after the focus is lost. The current call stack is as follows:
- The control gets the focus 
- OnGotFocus is hit
- OnLostFocus is hit incorrectly
- The focus is removed from the control
- The OnLostFocus is not hit and it should be

As a workaround you can hook to these events in the constructor of the custom ComboBox:

    public MyRadComboBox()
    {
        this.GotFocus += MyRadComboBox_GotFocus;
        this.LostFocus += MyRadComboBox_LostFocus;
    }

See the supported thread.

The problem is that when you have SearchMode=Contains , and have item that contains the entered text and one that starts with this text, the ComboBox selects the one which contains the text(as it is with smaller index) but scrolls to the one that starts with it.

When using similar code as below, RadComboBox does not select the specified item. If the DomainDataSource data was already loaded, the selection is successful.
<telerikInput:RadComboBox ItemsSource="{Binding ElementName=MineTypeDomainDataSource, Path=Data}"
  SelectedItem="{Binding Path=MineType, Mode=TwoWay, NotifyOnValidationError=true, ValidatesOnExceptions=true}"> 
 <telerikInput:RadComboBox.ItemTemplate> 
  <DataTemplate> 
   <TextBlock Text="{Binding MineTypeName}" HorizontalAlignment="Left" /> 
  </DataTemplate> 
 </telerikInput:RadComboBox.ItemTemplate>  
</telerikInput:RadComboBox>  
<riaControls:DomainDataSource AutoLoad="True"
  x:Name="MineTypeDomainDataSource"
  QueryName="GetMineTypesQuery"
  LoadedData="MineTypeDomainDataSource_LoadedData"
  DomainContext="{StaticResource CitationDBSvc}" />

Create a RadComboBox, bound to a collection of objects, implementing INotifyPropertyChanged. Use DisplayMemberPath. Select an item, then change the display member property of the selected item - the selection box is not updated.
As workaround you can use DataTemplate:
<telerikInput:RadComboBox HorizontalAlignment="Left" Margin="122,52,0,0" VerticalAlignment="Top" MinWidth="150" x:Name="radComboBox" telerik:TextSearch.TextPath="Name">
    <telerikInput:RadComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Name}" />
        </DataTemplate>
    </telerikInput:RadComboBox.ItemTemplate>
</telerikInput:RadComboBox>

Available in LIB version 2016.3.1017, it will be also available in the 2016 R3 SP1 release. 

Available in LIB Version 2016.3.1010, it will be also available in the 2016 R3 SP1.

The issue can be reproduced also with the standard ComboBox:
https://connect.microsoft.com/VisualStudio/feedback/details/663870/wpf-4-0-combobox-itemtemplate-and-selectionboxitemtemplate-when-itemtemplate-is-bound#details