For me it was if you swallow a plant seed, the plant will grow in your stomach and you will die from it. My dad was joking, but I was 5 and didn't know any better. I went on believing that until I was in my teens because things like that you don't re-evaluate every so often. It's just "my dad told me this and he wouldn't lie to me because grownups don't lie" stuck on autopilot.