This is a very strange behavior. Could you please provide some more info in order to help the investigation. What version of Visual Studio are you using- VS2010 or VS2008 and what version of controls. Also have you moved Telerik assemblies to another folder or installed some of our Latest Internal Builds, service packs, other Telerik products etc.. before the issue appeared.
In the meantime you can try not to just delete the tab with our controls, but to Reset the toolbox (right click on toolbox->Reset toolbox).
the Telerik team