The monster under the bed is a great example of an urban legend that just isn't true. This urban legend is told by parents who want their kids to stay in their beds so they won't get up. They say that something is under the bed. It also could be shadows the kid sees and thinks that something is there.
Bloody Mary is another untrue but believed urban legend. To "see" Bloody Mary, it is said someone has to flash the lights and flush the toilet a certain amount of times and she will appear in the mirror. Lots of people clam to have tried it and have seen her ghost but they have no proof because she is a ghost. Most people usually just do this prank to entertain their friends or scare them.
Lastly, the urban legend in To Kill a Mockingbird is Boo Radley. Everyone in the town of Maycomb believes that Boo Radley eats living animals. When he was little, he stabbed his dad because he was mad at him, but Boo just didn't know better. The truth is that he got angry at his father when he was little and stabbed him.
Most Urban Legends aren't true, they just give people things to talk about and are entertaining. The monster under the bed, Bloody Mary and to Kill A Mockingbird are all great examples of this theory.