5. Mouse slip
4. Forgot to call check
3. Accidentally touched 2 pieces, tried to fix it
2. Forgot to hit the clock button
1. Castle through attacked square
So, the only one of these that was an acual "illegal move" of the sort LLMs make was the castle through attacked square.
LLMs sometimes just move pieces wherever. And that does not happen when humans who know the rules play. Yes, they may mess up en passant or promotion too. But a basic "how a single piece moves" rule is what LLMs f up.
Moving through check definitely counts as as an example of a human knowing the rule and yet playing the move anyway. Which was the position you took when claiming humans would not do moves against rules they have learned.
In my experience sub 2000 players playing OTB informal chess do illegal moves fairly regularly, perhaps 1 in 50 games. Moving knights one square too far, slipping a bishop from one line to the next on a long diagonal. Castling after moving the king, not moving out of check, moving into check (especially by moving a pinned piece)
They all meet the criteria of knowing the rules and playing something else. Oftentimes people do this because they have a mistaken assumption about board state. I suspect the same is true for LLMs, they are making valid moves for what they mistakenly think the board is. That would be difficult to test, but I think possible with the right introspection tools.
