I saw one that was disguised as one of those dumb quiz things about identifying the subliminal messages hidden in advertising. It shows a few pictures of ads for a moment, and then highlights areas or rotates them to show hidden things in the picture. Then the last one says that it's a bit harder to notice, so they'll give you more time to study it in detail. After a few moments, pictures of aborted fetuses and suicide jumpers start flashing on the screen with this horrible screaming.