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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".

Very understandable how this developed in the era of over the board play. You can't have two queens when they don't physically exist!

I always use an upside down rook, somehow there's always one available by then.

I end up using a checkers piece, since pretty much every board I've played on has them.

It used to be a potential invitation for a fist-fight before we had pocket computers with which the aggrieved (and unaware) player could verify the move's legality.

This is true.

I've taken to asking players who are unfamiliar with the game if their familiar with it on occasion, at least ones who I expect will be more frustrated than amused.