I think it is a little unfair to start insulting Telerik as this issue cleary isn't something they have deliberately done, I mean look at the stack trace of the error you sent, do you see any Telerik code being referenced? Nope it is all from System.Web which comes from Microsoft so if you want to call anyone lazy start with them....
I too can't reproduce your problem so if you could kindly give a bit more detail on where you navigating, eg, within the admin seciton, on the live site using some sort of navigation that causes post backs (there is also hardly any ajax being used in sitefinity by the way), or navigating through the page editor?
If you want to check if it is in fact EventValidation that is causing your problem you can add to your web config this code:
You must put that between your <system.web> tags otherwise it won't work. That should at least help you rule out it being that if you are so convinced it is something else.
Hope that helps.