What is something you were just plain wrong about?
I was once told that hot water would freeze faster than cold water. I don't know why I believed it but I just commiteted it to my knowlege bank. Then one day, something came up in a coversation where I was able to use that fun fact... not only did I get a bunch of people looking at me like I was an idiot, but we proceeded to try the experience because I was convinced I was right. I wasn't.