I have a RadComboBox with "AllowCustomText" set to "True". If I haven't selected any pre-populated values, the combobox can accept custom text. But once I select any pre-populated values, it doesn't accept any new custom text. After I type something and press Enter, it just goes back to the previously selected value. This behavior is reproducible on the demo site:
http://demos.telerik.com/aspnet-ajax/ComboBox/Examples/Programming/ServerEvents/DefaultCS.aspx
http://demos.telerik.com/aspnet-ajax/ComboBox/Examples/Programming/ServerEvents/DefaultCS.aspx