Illegal Position Contest!

Why don't you put this in the shortest proof game thread without the knight?

Why illegal? White must play b3 to let the black king reach a3 on the way to h2, but before that the black knight must reach a1 or it couldn't get there.  (Black doesn't have enough captures for the knight to be a promoted f-pawn.) The white rook must leave the back rank along the h-file while the black king is coming across but has no way to return to the back rank after the king gets to h2.

Another try is cross-captures a2xb3 and bxa, so that the wR can get in via the open file. The NW cage unlocks after retraction of d7-d6, so that bBc8 was captured at home. Also, the bRa8 can only escape after axb6 but there would already be a bN on a8 to block the way. So whereas the f pawn has enough captures to promote on h1, there is not a second black piece on offer for sacrifice. 

White is missing three men, so after the captures by Black on b6 and f1, only one capture is left, and the f-pawn can't promote. Am I missing something?

I didn’t see bPb6 lol. Maybe remove the wNe8 and the position is still illegal.

yeah

Here's my first try at an illegality proof. The white rook on d8 was promoted after six captures by the h2 pawn. The f8 bishop was captured at home, and the h8 rook was captured on its back rank, but not by the promoting pawn, else the white king couldn't get in place on the 8th rank. So the six black men captured by the h2 pawn included the black a-pawn and c-pawn, which had to first promote after reaching a3 and c3 and making one capture each. The pieces they captured could only be the white a1 rook and c1 bishop, but then there's no way for the h1 rook to reach d1, since the f1 bishop never moved.

I was thinking the same until I realised the wRh1 can die at d6 (by the c pawn) with the wRd1 as the queen rook, whereas wBc1 is sacrificed for the black a pawn to promote.

Here is this game I just found to illustrate my idea

I was finishing up a proof game, but you beat me to it.

So first it was illegal, then it was legal, then it was illegal again, and finally it was legal.

That’s basically what happened lol. I knew cxd6 was a thing as well as promoting, so I thought the a and c pawn would have to share a capture between them, but the wRh1 still exists (as a capturable piece). Anyway, have you tried #6599? 

I haven't thought about it yet. I'll try later, but first I have to watch a football game.

Edit - Posted and deleted a faulty proof, still thinking about it.

Another try. The White king and rook at d2 have to be in place before the knight moves to e1, and it could only come from f3. (If it had come from d3 with the king at c1 and rook at d1, the last knight couldn't move to c1.) After the f2 pawn moves to f3 to let the black king move to the third rank, White has no more moves to let the black king move again.

Are you trying to say that the black king can only get in when f2-f3 is played? That is the purpose of the knights and rooks, but white still has moves. Have you considered the uncapture of a bishop?

Yes, I don't think the king can get in with the pawn still at f2. I didn't consider the uncapture of a bishop, since White's only promotion was to an extra knight, and White's original bishop had to be captured at b2 to let the a7 pawn promote, or else White wouldn't have enough captures to have an extra knight.

The bishop could also be captured at d6, but it is needed to give tempi for white at the end. And if the c pawn was captured at home, there is one capture left, but the queen and king rook have yet to be captured.

Well done! That was tricky.

I agree, it was tricky. That's what makes for a good illegal position challenge.

I was looking at some of the earlier posts in this thread and found #392, posted 5 years ago. I do not think anyone has attempted it after seeing the next few posts. It was a composition by @cobra91 and it really is a brilliant position to prove illegal. I have an illegality proof myself (which I think is sufficient) - but can you prove it illegal too?

Edit: Now with a black queen at c6.