Are you sure? :)
Most of the time, newer chess players are not familiar with some of the stranger-looking "special moves" that are indeed totally legitimate.
For instance, many players (even some who have been playing for quite a while!) are not familiar with the special pawn-takes-pawn move known as en passant (literally "in passing"):
...Or maybe a player isn't completely familiar with all the rules of castling to either side of the board. In this diagram, the Black pieces show the two possible arrangements after castling to one side or the other (no, it's not meant to suggest that it's OK to have two kings!), whereas the white pieces are used to show how the pieces move to castle:
Did you know that, although the King is barred from moving out of or through check during castling, it's totally acceptable for the Rook to do so?
keywords: cheating cheat cheated illegal impossible move