Things that most people believe that simply are not true:

1. You can boil a frog to death if you heat the water very slowly. NOT TRUE.
2. You only use ten percent of your brain. NOT TRUE.
3. You have to wait a half hour after eating before going swimming. NOT TRUE.

As Snopes explains about such things:

Regardless of the exact version heard, the myth is spread and repeated, by both the well-meaning and the deliberately deceptive. The belief that remains, then, is what Robert J. Samuelson termed a "psycho-fact, [a] belief that, though not supported by hard evidence, is taken as real because its constant repetition changes the way we experience life." People who don't know any better will repeat it over and over, until, like the admonition against swimming right after you eat, the claim is widely believed. ("Triumph of the Psycho-Fact," Newsweek, 9 May 1994.)