When there is a RadButton with configured ValidationGroup, the client-side ASP.NET validation on the page will not be executed, if there are validation controls, which use the same ValidationGroup. The unexpected behaviour is observed in FireFox and Opera.
I have the same problem.
Fixed? It doesn't fire the OnClick event yet.
It looks like this has been fixed in the final 2013Q1 release.
This issue BREAKS existing functionality as well as compatibility with standard asp.net button controls.