Take back Nxh7 (assuming the piece on h7 wasn't a rook or queen) and instead play Ng6#
Take back your "sorry move" - for once
Take back Nxh7 (assuming the piece on h7 wasn't a rook or queen) and instead play Ng6#
Good try! You saw that capturing was necessary, or black would have no last move. However if - before Nf8xh7 - there was a bishop/pawn on h7, then how do you checkmate and if it was a knight/pawn, then what was black's last move?
h7. how'd you get four knights?
The chess.com interface is so generous! It permits me to place 20 knights when I choose to.
The diagram and the solution conform to the "convention" that they must be legal, i.e. can occur in a standard chess game. Obviously, two of the knights then come from promoted pawns. A "legal chess game" need not be a "good chess game".

Take back Nxh7 (assuming the piece on h7 wasn't a rook or queen) and instead play Ng6#
Good try! You saw that capturing was necessary, or black would have no last move. However if - before Nf8xh7 - there was a bishop/pawn on h7, then how do you checkmate and if it was a knight/pawn, then what was black's last move?
If there was a bishop or pawn on h7 and the knight moved to capture then I'm not sure. Not seeing anything right off.
The other option I see is there was a black knight on g8 and instead of Nexg8, Ng6# instead.
The other option I see is there was a black knight on g8 and instead of Nexg8, Ng6# instead.
Similar problems with Ne7xg8. Whatever you capture, either white cannot mate in one, or black had no last move. Keep looking!
@ Arisktotle I don't see why Martins take back Ne7xng8 to Ne7 to g6# Couldn't the move previous to this been white rook g8+ black nh6xg8. I looked again I would have been in check.Nice puzzle.
@ Arisktotle I don't see why Martins take back Ne7xng8 to Ne7 to g6# Couldn't the move previous to this been white rook g8+ black nh6xg8. I looked again I would have been in check.Nice puzzle.
Right, you got it! There is a similar argument against the last move Nf8xNh7; black's previous move would have been check as well.
Official solution (also given by winterberger):
The last move was Qc8xRb7; instead white could have checkmated with Ng8-e7#!
All other last moves fail because either (a) there is no follow up mate, or (b) black has no legal last move before white's last move! Convention requires there must always be at least one legal chess game for the diagram plus the solution!

one more solution is take back Qa8xRb7 and play Ng8-e7#!
Not quite, that would be check, not mate

h7. how'd you get four knights?
The chess.com interface is so generous! It permits me to place 20 knights when I choose to.
The diagram and the solution conform to the "convention" that they must be legal, i.e. can occur in a standard chess game. Obviously, two of the knights then come from promoted pawns. A "legal chess game" need not be a "good chess game".
oh. touché.
.
For once, you are allowed to take back a move you regretted. Not just that, but as an early Christmas gift, you can take back any move you like! One condition though. Having taken it back, you must replace it with a move which immediately checkmates black. Not that difficult, right?
Please post your answers!