YouTube April Fool's Prank
Click on the links below to see what happens when you add &textp=fool to a YouTube URL:
With:http://www.youtube.com/watch?v=Tim5nU3DwIE&textp=fool
Without:http://www.youtube.com/watch?v=Tim5nU3DwIE
Unfortunately, that doesn't appear to work with embedded videos, just on the main YouTube pages.
(I would have posted this much earlier, but I don't have LJ access at work anymore.)