Given this definition:

<ig:XamComboEditor 
  x:Name="productCombo" 
  ItemsSource="{Binding Products}" 
  SelectedItem="{Binding Product, Mode=TwoWay, ValidatesOnExceptions=True, NotifyOnValidationError=True, ValidatesOnDataErrors=True, ValidatesOnNotifyDataErrors=True}" 
  DisplayMemberPath="ComboDisplayName" 
  HorizontalAlignment="Stretch" 
  Margin="0" 
  VerticalAlignment="Center" 
  AutoComplete="True" 
  CustomValueEnteredAction="Add" 
  DataObjectRequested="productCombo_DataObjectRequested"/>

I get this exception trying to add an item not currently in 'Products' ie as a consequence of the CustomValueEnteredAction.

A first chance exception of type 'System.Exception' occurred in InfragisticsSL4.Controls.Editors.XamComboEditor.v10.2
System.Exception
Can not create a new data item, when there is no ItemSource set. Please make sure that you have hooked up an ItemSource to the XamWebComboEditor.
   at Infragistics.Controls.Editors.XamComboEditor.CreateItem()
   at Infragistics.Controls.Editors.XamComboEditor.CommitTextAsSelected(Boolean closeDropDown, Boolean cancelSelection, Boolean moveSelectionToTheEnd, Boolean allowUseOfFocusedItem)
   at Infragistics.Controls.Editors.XamComboEditor.Editor_LostFocus(Object sender, RoutedEventArgs e)
   at MS.Internal.CoreInvokeHandler.InvokeEventHandler(Int32 typeIndex, Delegate handlerDelegate, Object sender, Object args)
   at MS.Internal.JoltHelper.FireEvent(IntPtr unmanagedObj, IntPtr unmanagedObjArgs, Int32 argsTypeIndex, String eventName)