(Un)Fortunately I've never had this happen in any of my schools. But one of my teachers, while looking for a Bill Nye episode, accidentally showed a youtube poop. I couldn't stop laughing.

This is the most frustrating thing in the whole fucking universe. Especially since you can't really warn them without disrupting the whole class. PLUS, even if you do go ahead and warn them, they don't always correct the mistake in time.