There seems to be an issue that renders embedded videos from both youtube and vimeo to playback in fullscreen mode directly from the forum. The issue is that when a video starts playing and I click on the fullscreen icon, the video briefly enters fullscreen then immediately snaps back to the forum but in a different location. The video seems to keep playing although you have to find it in the thread again. Weird.
I've noticed this on 2 different OS X systems using Safari and my work craptop running Windows 8 and IE.