Illegal Position Contest!

I usually start with a main idea/structure that I want to implement on the board. For example, I had this pawn structure which featured a narrow bishop passage in one of my previous positions (#8765), and the idea was to prevent the bishops from swapping places. After this, I fill the board with pieces to limit the number of captures for each side, tightening the inventory.

In some of my positions, I try to make one/both sides have limited tempo - in this case, I trap the other pieces on the board to prevent them from providing tempo moves. Here, the closest tries to legality typically require certain "events" to occur when retracting moves (e.g. un-capturing a certain piece, retracting a pawn back a certain distance).

For my other positions, the illegality is more focused on certain pieces and pawn structures. This often involves some moves that must precede others (e.g. b3 or d3 must be played to free wBc1), creating a chain of moves one before another. The illegality would then be caused when the chain circles back on itself, creating the paradox of "which came first".

You've also created some tricky positions where the solver has to prove a required castling would not be legal. I wonder if the same thing could be done for an en passant capture. I don't recall seeing one like that.

Or make those two themes the contradiction itself that makes it illegal. Maybe a position where en passant could be legal and castling could be legal but not both at the same time, if this were somehow possible.

Well, this is what I came up with. Either black can capture cxd3 e.p. or white still has their queenside castling right, but not both.

I've seen a number of published studies based on the interaction between en passant and castling, but of course in those the positions were all legal.

Here is a rather silly problem I came up with. When a game reached the position below, a spectator reported that White had just moved a pawn and said "Checkmate". Black replied "That's not mate", made a move, and said "Checkmate".

If the spectator's report was accurate, can you determine which player made an illegal move?

Black just played 0...f4xe3 ep, preceded by -1.e2-e4+

With pawns on e2 and g2, and all 8 white pawns on the board, white must have illegally jumped his bishop over one of the pawns en route to b5.

I totally missed the concept of the bishop getting out, was looking at the pawn doubling for a few seconds but yeah f and h pawns could have easily reached their current squares so that the e2 pawn was at home, but yeah then the b5 bishop couldn't have gotten out.

Either player might have made the illegal move. If the white pawn reached e4 from e3, Black's ep capture was illegal.

yes

Can’t this position be legal if you play Nf3 e5 Ng1

How?

[Hint: Remember En Passant ]

white king under max checks( and pawns on 8th and 1st rank for the funny)

Easily legal.

More like: remember the Bishop on e5! [Smullyan's composition, mirrored]

Legal. Last moves -1.Kg2 h4

It's a little trickier with BigDoggProblem's correction.