My brother was flipping through channels one time when I was about 5 or 6, and for a few minutes he left it on a horror movie called Squirm. Some people were fishing and the worms burrowed into one guy's face. It was terrifying, but my brother was laughing, which just made it all the more disturbing to me. That image haunted me for years when I would try to go to sleep.
I later found out that my brother was laughing because the movie is terrible. It was even featured on MST3K at one point.