Recently I played in a tournament where at one point my opponent made an illegal move. They had inadvertently hung their knight and after I captured it with my rook, my opponent tried to capture it with a pawn, however I had this pawn pinned to their king. As they tried to make the capture I pointed this out to them and they sat back (a bit dismayed) to find another move. I know that in this case I could get time added to my clock, but this wasn’t a concern as I had an edge in the position before this blunder and had plenty of time on the clock. As I was looking over the game later with a friend of mine he said that what he likes to do when this happens in non-tourney games is to find the move that threatens the most material, regardless of if it hangs a piece, for when your opponent attempts to capture you point out that they are in check and must verify this first. For example in the position mentioned above if I had allowed the rook capture I could have moved my queen to threaten his queen, then if they had moved away I could point out the check which they would have to defend against at which point I could take their queen.
What is the ruling about this? If two players “play through a check” in a game what happens when they notice it later? Could this “trick” be used in a tournament game, even if it is unsporting?
It can (and often does) get more complicated than this, but when there is an illegal move detected several moves after the fact, if the players have scoresheets the position should be restored to the point where the (first) illegal move was made and a different but legal move made. Touch move still applies, if such a legal move exists.
Not unless your opponent had already pressed his clock after making the illegal move.
In the situation you describe, any sane TD would require the game to revert to the position just before the illegal move, and require the player who made the illegal move to make a legal move with the same piece (if such a move exists). In your example, if your pin of his pawn was along a file, the pawn might be legally able to move straight forward, and the TD would likely require your opponent to do so.
I’d say that knowingly allowing your opponent to make an illegal move would be extremely bad sportsmanship, to say the least.
Bill Smythe
After 10 moves by both players the illegal move stands. Before that, one of the players would have to claim that an illegal move occurred. I am summarizing here, it’s covered in great detail under rule 11 in the USCF regulations. You can also find lots of related discussions on the Chess Tournaments forum here.
google “illegal move” site:uschess.org
Of course this trick could be used against you, so watch out! Like the instructions to the boxer, “Protect yourself at all times.” In other words, even though a cheap shot is against the rules, you shouldn’t ever let your guard down, not even between rounds.
If I were you, I would keep a wary eye on this “friend” from now on, even away from the chessboard.
Back when I was still a TD, I had to handle a funny incident.
Expert vs Class A, no time pressure, TD (me) is observing the game closely. White made a check. Black didn’t notice, and answered with a check: double-check! White thought for quite a while, then used his checking piece to block black’s check: double-UN-check!!
Even though strictly speaking I am supposed to wait for a claim, at this point I stopped the clocks and made them back up a move. My explanation was that there was no way either player was ever going to reconstruct what just happened. I left the clocks as is, their penalty for making an illegal move. Both players seemed satisfied that I made the right decision.
How do you figure? Since there was no time pressure, Rule 11A says otherwise.
Alex Relyea
You must mean 11H. This was an 11H1 situation, so I was supposed to wait. (I’m looking at the 4th edition, I hope the numbering scheme is unchanged.)
Yes, I must have meant 11H. I’m not sure what I was thinking. In any event, I don’t use 11H1. It seems to cause confusion. If you use 11H1 do you have to announce it before the tournament?
Alex Relyea
It’s not 11H1 in particular that causes confusion. It’s the rulebook in general. On the whole, I find the USCF rulebook badly done, and getting worse over time. But the absolute worst is listening to TDs go on for hours about what the rules actually are. Those two together are why I don’t want to be a TD anymore.
Nope. I’m just a player, I don’t have to announce anything.
an oxymoron?
Hey, now… I like to represent that remark. 
Personally, if I find players intentionally allowing illegal moves/making illegal moves to gain an advantage–I come down pretty hard on them (time penalties, even game forfeits in extreme cases). Same thing with the announcement of check. I’ve had kids tell me, “but my Coach says I should never say ‘Check’ because then I can make them move something they don’t want to,” (true story–from this past weekend’s Supernationals, an Unrated section, even), and I have very little tolerance for such teachings, or such Coaches, for that matter. [Thumbing through my rulebook, desperately seeking a way to prevent and/or sanction bad Coaching…]
I remember GM Edmar Mednis at a Chess Camp reminding everyone that it was impolite to announce check, even insulting to your opponent to assume he did not see it.
A clarification, I suppose, is in order. This is one of those instances where level and intent make a significant difference, even though the action may be the same. In “Adult” or Professional chess, of course, it’s rare and some consider rude to announce check. My problem is with the “Scholastic” ranks, where that teaching is misconstrued to give a young and impressionable player a legalistic grounds for winning by unethical means (if they touch a piece that is thence lost because they have to move it, and still get out of the check when it’s pointed out). I feel it’s a very cheap way to win the game, as the contest becomes no longer about ideas and better play, but about rules-managing and claim-appropriation. Just my perspective on the situation.
If a scholastic player is also going to play in serious tournaments, he should expect to know if his King is en prise, even if his opponent doesn’t remind him.
Adult players should expect to know when their king is en-prise without being told. Then again I’ve pulled a few rabbits out of my hat with 1 second left, because my opponent overlooked his hanging king. 