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.
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You know, as many reports as there are, searching for related bug report before reporting yourself just isn't an effective option in most cases. Sorry for the duplicate.
Don't worry, before I changed the title you wouldn't be able to find it.
Duplicate of MC-4028.