Penny on the train tracks
As a kid, did someone warn you that putting pennies on train tracks would derail a train? We definitely heard this bunk and believed it. But it's probably a good idea to scare children so they don't do stupid things and become flattened by a train when they are just trying to create an awesome squished penny for their coin collection.
People still seem to search Google about the penny derailment, but Snopes is on the case and confirms this is definitely false. They do have a nice list of people who have been killed while trying to put pennies on the tracks. Whoops.
Photo by elisfanclub, more photos on Flickr.