Here's a two-fer, because I just came across this startling blunder.
In the 1981 Uzbekistan Championship, White (Dekhanov) resigned to Black (K. Yusupov) in a
won position after Yusupov played Qa6. Dekhanov thought that the risk of Qf1+ meant an exchange of queens would be forced, leading to a loss in about another thirty moves.
However, he missed a forced mate in four, from the board position given below.
Solutions:
1. g4+ fxg4+
2. Nxg4+ g5
3. Qe8+ Qg6
4. Nf6#
or
1. g4+ fxg4+
2. Nxg4+ Rc5
3. Nf6+ Qxf6+
4. Qe2#
Another nice thing about this mate is that the loser gets to put his opponent in check for a little bit as part of the checkmate. That's not seen very often.
The FEN for this puzzle is:
8/pp6/q5pp/1Q2Np1k/5P2/P5PK/1Pr4P/8
If you paste it into
Chessup, you can work through the problem there.