Originally posted by David113Very good. Few people were able to solve this one! It was entered in a live solving tournament, and only GM solver Michel Caillaud got the solution, albeit after time expired.
Maybe like this?
This position is illegal because before Qh5+ black has no last move (the only possible last move is a capture on b5, but then where is the captured piece?).
It is clear why removing the bB, bP or wQ makes the position legal.
Without the wB the position is legal because the last moves were 1...Kx ...[text shortened]... ecause the last moves were 1...KxRb5 (the rook isn't reborn because the wQ is on h1) 2.Qh1-h5+.