For the following position to occur in legal play, with Black on move, what would White's second-to-last move need to have been? Note that Black is checked by two pieces on the h1-h8 file and two more on the c1-h6 diagonal, and both the d2 bishop nor the h2 must have been at their present locations when check was given.
b/7P/6Pk/5P/4P/2PP/3B2KR/2Q1N2R w K - 0 1
This is my first effort at a retrograde analysis puzzle; I think the position shown is reachable via legal moves, but that there's only one possible way the last three half-moves could have transpired. Did I formulate the puzzle "correctly"? Could/should it be improved? Would it be better with fewer pieces, e.g.
8/8/4Pk/3P/8/8/5QK/B w K - 0 1
In the latter formulation, I like the way the white queen is placed so that it could have moved from b2 or d4, unblocking the bishop, but for the fact that it would have been giving check if it had been in either of those places.
Here's yet another formulation, perhaps a bit more subtle, but I think the solution is still unique. Note that in this revision, White's last move is not certain, but the second-to-last move is.
[FEN "8/8/4Pk/8/8/8/8/B6K w - - 0 1"]