Illegal Position Contest!

In response to #8415 posted by @EvinSung

board flip

Only possible knight check is an underpromotion, but then white doesn't have a legal last move. All other pieces stuck, and bishop d2 means black would have been in check. Couldn't have been d2xc3 either because then the bishop could never have gotten to e1. Finally, couldn't have been black pawn d2 X c1 = Knight because whites also in check by the bishop so the pawn would have to promote from c2. I like this one.

That is correct! Well done!

#8434 should be illegal, black had to make 2 captures, both on light squares (one to promote on f1 and another to capture on c6, but the only two pieces white is missing are a queen and a dark squared bishop.

Illegal. The white queen had to be captured twice, once on c6 and once to let the g7 pawn promote to a light-squared bishop. (If the pawn captured on h2, the promoted bishop could never leave the 1st or 2nd rank.)

Actually the bishop could get out, it just couldn’t get to d5 to the c6 pawn.

just wordering, is this illegal?

and i am a legal post

This is rather easily legal. Me8vVv might want to post it in the Shortest Proof Game thread.

Yes, it's obviously illegal. White has gotten a doubled pawn without making any captures.

Good start @Chesspro5747 and @n9531l1, but there’s more to the illegality proof than that. In fact, the promoted bishop can reach d5, yet the position is still illegal. For example, why can’t the promoted bishop escape via b3-c4? Or escape via c6, before bxc6 happens? Here’s a diagram to show what I mean (keeping only the relevant pieces):

I think I see. In the main variation, the c1-bishop could never have moved and therefore could never have been captured on h2. In the second variation, there would still be a black bishop on c8, and it could never get to d5 nor h5.

Behold, this abomination

No pieces were captured and we have 15 necessary captures for both sides.

Nice finish! Maybe I gave away too much with the board…

The main idea was that the black light-squared bishop could get to d5, and the promoted bishop could also get to d5, but never both.