I remember a few when I was a kid playing my dad who used to be better than me but now glad to say i've overtaken him.
Bear in mind this is a kid's point of view.
1. Thought you could put your newly promoted queen anywhere. It happened cause I promoted to a queen and my dad took it. Upset by this I said "what's the point in me changing it if it just gets taken?"
dad" don't put it there then!"
"ah, didn't know you were allowed to put the new queen on any empty square" So I did for a while in future games
2. Playing my uncle and his pawn hit the 8th rank. "Can I have a rook back please?"
Thinking that you were only allowed to get a captured piece back. I expect this is quite common.
3. Uncle playing me and I had only the king left. "I win"
"i'm safe where I am"
"but you have to move so you've lost"
Turns out I was right but didn't know it.
4. Playing my dad and we had a chess clock. He has just his King left but I have a Bishop and a King. We play on to see who runs out of time and we both still think we can win if the other runs out of time.
5. Left the king in check at school and didn't notice. Opponent just takes my king, I see ah that must mean i've lost then. game over