Minecraft, in slow motion. This one is so strange I can't really explain it beyond that. I think it's best to just watch the video.

http://www.youtube.com/watch?v=3fi58ITZaPM

I wouldn't normally be reporting something I wasn't able to at least reproduce, but my friend recently confirmed this was happening to him, but he doesn't have a Jira account, and this is the kind of bug I'd rather risk reporting a duplicate than not reporting at all.