http://en.lichess.org/training/35232
After Qe3+ Kd1 Bb2 c5 Qd3+ Ke1 Qxb1+ Qd1, black is fully winning, up a piece and with the ability to trade queens and simplify. I played Kf8 to start centralizing my king, and force white to trade queens. That failed the puzzle for no reason, as it is -4.7 in the evaluation and very clearly winning!
The puzzle wanted the obscure Qc4 which the computer likes slightly more (it evaluates to -5.4) but it leaves the queens on the board and does not simplify, resulting in a significantly harder endgame for a human to play. Nor does it immediately win any more material.
If puzzles accepted as alternate moves any moves over an evaluation threshold such as 3.0, this and thousands of other puzzles would be fixed.
After Qe3+ Kd1 Bb2 c5 Qd3+ Ke1 Qxb1+ Qd1, black is fully winning, up a piece and with the ability to trade queens and simplify. I played Kf8 to start centralizing my king, and force white to trade queens. That failed the puzzle for no reason, as it is -4.7 in the evaluation and very clearly winning!
The puzzle wanted the obscure Qc4 which the computer likes slightly more (it evaluates to -5.4) but it leaves the queens on the board and does not simplify, resulting in a significantly harder endgame for a human to play. Nor does it immediately win any more material.
If puzzles accepted as alternate moves any moves over an evaluation threshold such as 3.0, this and thousands of other puzzles would be fixed.