Two things. My brother thought "gaining" and "gagging" were interchangeable, but he'd use the wrong. For instance, running ahead of me down the stairs he would shout back, "I'm gagging on you."
Up until just a few years ago, I'm 29, I thought cartwheel was named after a gymnast rather that it being named after how it looks when you do it comparing to an actual cartwheel. In my defense there are a lot of gymnastic and skating moves named after the first people who did them, but that doesn't stop friends and family from chuckling, "Oh, like Bob Cartwheel?"
A childhood belief about speaking

others believed things like this
- i used to belive that if i fell on my head my head would come off while i was doing a cartwheelAnon
- when i was growing up i had two imaginary friends. One was named Jennifer, the other was named Onion.— Tessica
- I used to believe there was a Russian Leader named Garbage Trough and an Arabian named Are A FatAnon
- The boy in my class named "Sahand" must have a sister named "Safoot".Anon
- My 18 year old brother thought the former vice president was named Al Gorbachov.— Jeff
- I thought Kaa's name from The Jungle Book was spelled "Car" and that he had a brother named Truck.Anon