Also true for some other little rules people often forget, like underpromotion, castling through/out of check, stalemate, perpetual, 50 moves, etc.
Plus the general topic of how to use clocks.
Chess is so popular that most people first learn to play from parents, relatives, etc., so they might not have learned the full rules if they aren't interested in learning the game more seriously.
You say underpromotion, but something people are surprised by is making a second queen ime. People sometimes assume promotion is actually "taking a captured piece back".
Plus the general topic of how to use clocks.
Chess is so popular that most people first learn to play from parents, relatives, etc., so they might not have learned the full rules if they aren't interested in learning the game more seriously.