Before anyone else decides to rant about the infamous Code Geass chess match, let me just put this out there. Yes, Schneizel made an illegal move. He knew that the game was going to end in a stalemate, so he deliberately made the move to see how Zero would react. That way, he gains insight into his opponent's character rather than just ending the match. Zero, in turn, doesn't just end the game because he sees that as submitting to Schneizel's methods.

Yes, in real life he would've gotten called out for being a tool, and no, the show's depiction of chess isn't accurate. It's an anime for god's sake, you have to expect these things.