This happens me very frequently that I watch a video on Youtube (not full screen) and suddenly for some reason the video grows 2 or 4 times its size, but the box in which the video is displayed on the page stays the same size. This means I see only 1/4th of the video, because that's what fits into the box and I see its top left part only, therefore I'm unable to access the video controls either, because they are at the bottom of the video.
The way I usually fix it is reloading the youtube page and quickly pressing the full screen button, before the video starts, so it has no time to "grow out" of its displaying box, so I can watch it normally in full screen. But if I switch back from full screen then the sudden growth effect usually happening again.
Does anyone else experience this? I use Opera/Windows for browsing. This may be relevant, I don't know.