I've downloaded the latest Wiideo Center. I found there is a problem with many x264 files (arrow.103.hdtv-lol.mp4 for example). The built-in Mencoder does it and I tried a few other versions to no avail.
I've attached screenshots of G-spot and how the bug looks like in Firefox. Also, there is a .torrent file for "arrow.103.hdtv-lol.mp4" so the bug can be re-created.