Don't do it. This is what happens.
I did it by accident. When I got my first one, I wanted to test it out so I plugged it up to a 3s pack not even thinking that it would be to much.
Everything still works...including the BEC, but it looks ugly. Had I not realized it right away, who knows what would have hapened. This happened just sitting on the bench. Wasn't even running it and it overheated the BEC.