Movie Rule #6639: Garbage disposals will always attempt to mangle people's hands.
It was actually my nephew who brought this one to my attention. He's ten years old and when he came over, he had never seen a garbage disposal in real life before. When he saw it, he asked, "Does it ever chop up people's hands?" At first, I was horrified, like "No, why would you think that?" Then he said, "In the movies, garbage disposals always chop up people's hands."
Which is true. Especially in horror movies. Someone scrapes something into the garbage disposal and hits the switch. It doesn't work, hmm. Or they drop something into the garbage disposal. So they reach down into the drain. Then the possessed house or the ghost or the computer virus or the blob hits the switch and then there's screaming or blood or all of the above. Or the disposal goes off a split-second after the person manages to pull his/her hand out. Which, of course, rarely happens in the real world. I personally am not stupid enough to reach into a garbage disposal, and even if I was, the odds of it coming on spontaneously are slim-to-none. But in movies, it happens every time. When was the last time you saw someone in a movie scrap some chicken bones into the sink, hit the switch, grind up the bones, and walk away? I'd like to see that.
Examples: Final Destination 2, Heroes