Available in the 2016 R3 Release.

Please check our help article for more information: 
http://docs.telerik.com/devtools/wpf/controls/radcombobox/features/multiple-selection

According to the following article, when the UpdateSelectionOnLostFocus property is set to True and you enter a text that cannot update the selection, once the control loses the focus, the typed text should stay in the textbox. 
https://docs.telerik.com/devtools/wpf/controls/radcombobox/howto/perform-selection-only-lost-focus

However, this doesn't happen if an item is already selected. In this case, the searched text is cleared. If you do not have a selection, then the text stays on lost focus. 

To work this around, you can cache the value of the RadComboBox's Text property in the PreviewLostKeyboardFocus event handler and then restore it in the LostFocus event handler. 

private string textCache;
private void RadComboBox_PreviewLostKeyboardFocus(object sender, System.Windows.Input.KeyboardFocusChangedEventArgs e)
{
	this.textCache = this.comboBox.Text;
}
private void RadComboBox_LostFocus(object sender, RoutedEventArgs e)
{
	this.comboBox.Text = this.textCache;
}

Combobox with 2 items -- "Item1"   and "it": typing "it" selects the 1st item which is "Item1" and it should select the second one which is "it"

When the app is in full screen OOB mode the placement of the drop down portion of the ComboBox control is incorrect.

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

IsEditable=True and ReadOnly=True

RadComboBox and RadRibbonComboBox in their NonEditable mode do not reflect background set inline. A style for the ToggleButton contained in the NonEditable templates prevents the TemplateBinding from working properly as it uses a hard-coded background. For RadRibbonComboBox it is a Metro theme-only issue.

Workaround: use OnDropDownClosed event.

RadComboBox with IsEditable = true, IsReadOnly = true and IsFilteringEnabled = true. Select an item and then in order for the filtering to work you have to delete the selected text with Backspace

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)

Enable the ComboBox to open its DropDown in up as well as down direction

The same issue can be observe when placing the control inside ToolBar.

After making a selection in the control and changing the theme two times in a row an InvalidOperationException is thrown about the animation.

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}" />