Illegal Position Contest!

Has anyone else got this error? I tried refreshing multiple times and it comes up with the same message.

I haven't seen that kind of error.

The position in your post appears to be legal. Was it meant to be an illegal position?

Legal.

This was my first attempt at an illegal position due to tempo. But if you’ve got a proof game then I guess it’s legal.

Edit: just realised the wRd8 can move to make way for bNb7. Swapping squares with bBb6 and bQa6 might do the trick.

Edit 2: None of this matters because white can retract hxg. I’ll need to find a way to fix that.

Does this work?

nKB5/1nrp1pp1/qrRp4/bkp5/pp6/R2P1P1P/PPP2PP1/4N2b b - - 0 1

I'll think about this position (ignore the move).

Now I get why you made the move. Before we could post positions without needing a move. What happened?

You know as much about that as I do.

This one seems illegal to me. Two captures by the black h-pawn to promote and one by the black e-pawn use up all of Black's captures. The bishop at h1 can unpromote and retract to h7, but then Black has no more retractions. The b7 knight can't uncapture a white piece (too many captures). Retracting the black king to c4 means putting the d3 pawn back on d2 and capturing the c1 bishop at home (too many captures). An uncapture by the pawn at d6 requires capturing the f8 bishop at home, but White's only capture was made by the e-pawn.

Adding on to that, Bc1 could be uncaptured by promoting black pawn on g5, requiring 1 move from c1. As well as the wRa3 which needs 7 moves to get there, there is a tempo shortage from black after white played d2-d3+, as only 6 tempi is left for black to push that pawn.

I missed that. The real problem with the black king retraction is that the white bishop and rook are both trapped. If White could make two moves in a row, I think it could be reached legally. Or if the rook were on c3.

I would call this (#6577) an excellent illegal position challenge. It reminds me a little of this old problem, where Black is one tempo short of being able to release a locked position.

Technically a3 is illegal because that position (with bPs from a3 to e3) is only legal with white (I meant black) to move. I’ve seen how this problem works and I was amazed at how black can’t use g6 to free his DSB, forcing e6 and wasting black one tempo.

The position given is illegal, but if the pawn is placed on a3 instead of a4, the position is legal. That position (with bPs from a3 to e3) is legal, and is reached with Black to move.

The white king rook had to go to h2 so the black king could come through on f2 to reach d1. If the black knight was on g1 at the time, it couldn't move again and the rook couldn't come around to f2. If the knight wasn't already on g1 when the king came by, it could never get there. 

Did I forget anything?

Without the knight a proof game would be easy but finding the shortest one would be hard.

The knight could unpromote

I didn't consider that possibility. But if the knight promoted after the king reached d1 and the white rook came around, a white knight would have to go to g1 to be captured and there's no way it could get there.

That’s all possibilities considered. Well done!