Illegal Position Contest!

This position is illegal with white to move.

Then I must have done something illegal when I reached it with White to move.

Dang it. I forgot about a knight going to f8. The idea was supposed to be that if it was white to move, then what was blacks last move. It couldn't have been with the pawns still on the 7th rank or the locked in bishop, so it had to be with the king or the pawn. I thought I had set up a position where blacks king couldn't have been in check the last move as a double check or discovered check would have been impossible (I was wrong lol), so the only possible last move had to be with the b pawn. But then how could White's king have gotten to a8 if the pawn was on b7, as a king can never reach the 8th rank with b,c,f, and g pawns still on the board. Let me try to correct it.

white to move

This position doesn't give a side to move at the end. That's a good thing, because specifying the side to move at the end of a proof game is a violation of common courtesy. But unfortunately, the position is legal and doesn't belong in this thread.

Totally illegal.

Too easy. No black king.

7590 is not legal. What was blacks last move?

white or black

In 7590, Black's last move had to be with the king or b pawn. But any square the king could have come from would have been checked multiple times in a way that's impossible. So it had to be with the pawn, but if it was with the pawn, how could the king have gotten to a8 with pawns on b7,c7,f7, and g7? A contradiction.

what if it is black to move

Then it should be legal.

#7590 is legal. Black's last move was Ke7-f8, followed (perhaps) by White's last move, Pd5-d6.

See #7593.

And how did the pawn check the king while the rook was already checking it?

Legal

Black only makes 15 captures, and white has 15 expendable pieces:

Just want to bump this one cause I still think it's the best one ever!