I used to think "understood" meant you didn't understand, because "under" means down. My mom would scold me and say, "Am I understood?" and I'd sheepishly reply "no", which got me 5 minutes in the corner every time for being a "smartmouth". One day, I heard someone use "understand" and "understood" in the same sentence, and it clicked.
A childhood belief about speaking

others believed things like this
- I used to believe the D in the Disney logo was a backwards G. I never understood why.— Nancy
- I used to believe that which ever way you looked it was north, never understood how people got lost :-S— Simon
- I thought drug stores sold drugs. I never understood how they could sell illegal drugs.— Diana Rios
- For many years I didn't understand why everyone was talking about the 'youth in Asia' (euthanasia).Anon
- Mom couldn't understand why at dinner time I wanted Chicken Pox.— Kristin
- I didn't understand about boiled eggs until my Mum explained I wasn't supposed to eat the shell as well.— Chris