This indeed was a known bug in the TabControl - one of the bindings in its ControlTemplate resulted in this error.
It should not be there any more with the SP release of the controls expected tomorrow.
Yes, this does not affect how the controls works.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items