[grumpopotamus]

I am an expert level chess player and I have multiple people around my level play illegal moves in classic time control games over the board. I have also watched streamers various levels above me try to play illegal moves repeatedly before realizing the UI was rejecting the move because it is illegal.

[zoky]

I’ve been to many USCF rated tournaments and have never once seen or even heard of anyone over the age of 8 try to play an illegal move. It may happen every now and then, but it’s exceedingly rare. LLMs, on the other hand, will gladly play the Siberian Swipe, and why not? There’s no consequence for doing so as far as they are concerned.

[Dr_Birdbrain]

There are illegal moves and there are illegal moves. There is trying to move your king five squares forward (which no amateur would ever do) and there is trying to move your King to a square controlled by an unseen piece, which can happen to somebody who is distracted or otherwise off their game.

Trying to castle through check is one that occasionally happens to me (I am rated 1800 on lichess).

[CooCooCaCha]

This is an important distinction. Anyone with chess experience would never try to move their king 5 spaces, but LLMs will do crazy things like that.

[dgfitz]

Moving your king controlled by an unrealized opponent square is simply responded to with “check” no?

[james_marks]

No, that would break the rule that one cannot move into check

[dgfitz]

Sorry yes, I meant the opponent would point it out. I’ve never played professional chess.

[umanwizard]

Sure, the opponent would point it out, just like they would presumably point it out if you played any illegal move. In serious tournament games they would probably also stop the clock, call over the arbiter, and inform him or her that you made an illegal move so you can be penalized (e.g. under FIDE rules if you make an illegal move your opponent gets 2 extra minutes on the clock).

That doesn't change that it's an illegal move.

[jeremyjh]

I'm rated 1450 USCF and I think I've seen 3 attempts to play an illegal move across around 300 classical games OTB. Only one of them was me. In blitz it does happen more.

[WhyOhWhyQ]

Would you say the apparent contradiction between what you and other commenters are saying is partly explained by the high volume of games you're playing? Or do you think there is some other reason?

[da_chicken]

I wouldn't. I never progressed beyond chess clubs in public schools and I certainly remember people making illegal moves in tournaments. Like that's why they make you both record all the moves. Because people make mistakes. Though, honestly, I remember more notation errors than play errors.

Accidentally moving into check is probably the most common illegal move. Castling though check is surprisingly common, too. Actually moving a piece incorrectly is fairly rare, though. I remember one tournament where one of the matches ended in a DQ because one of the players had two white bishops.

[ASUfool]

Could one have two white bishops after promoting a pawn?

[umanwizard]

Yes it's theoretically possible to have two light-squared bishops due to promotions but so exceedingly rare that I think most professional chess players will go their whole career without ever seeing that happen.

[da_chicken]

Outside of playing a game for piece value? No, not really.

In this case, of course, someone moved their bishop from black to white and their opponent didn't catch it until awhile later.

[IanCal]

Promoting to anything other than a queen is rare, and I expect the next most common is to a knight. Promoting to a bishop, while possible, is going to be extremely rare.

[nurettin]

At what level are you considered an expert? IM? CM? 1900 ELO OTB?

[umanwizard]

In the US at least 2000 USCF is considered "expert".