[SamBam]

Honestly, I made it make an illegal move in my very first game, in the third move. You just have to do stuff no normal player would do:

> You are a chess grandmaster playing as black and your goal is to win in as few moves as possible. I will give you the move sequence, and you will return your next move. No explanation needed.

1. b4 d5 2. b5 a6 3. b6

> bxc6

That's obviously illegal.

... to all those who are saying "well even good players can make illegal moves sometimes," that's just ridiculous. No player makes illegal moves that often.