I have never seen this error before. Due to the nature of when it is reported, I suspect that you may have a stuck key or something. I asume you have tried restarting your machine to ensure that is not the case. If not, I would try that first. If you have stuck key, then you should see a blue screen when you restart. If you don't suspect a stuck key, perhaps your last drag operation is somehow "stuck". Does this still occur with a restart of Studio?