Intricate chess rules

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I_say_hello

Please post some of the lesser known but important rules which can come to use in tournaments. Maybe you may get a win/draw in a losing position if you know the rules😀.

For example, in tournament chess, if we touch the rook first and then the king while castling, we are not allowed to castle and must move the rook.

Or you can post about the penalties for making an illegal move.

Note I have gone through the Fide rules but they are just too complicated...