Qf5 is a major blunder. Can you spot why? (Answer in comments)

That's because after the best move (Qxf5) black can't recapture the queen. If they do (as was the case for my opponent) there's simply Bxb5, which is a double check and mate.

After white takes the queen the best move for black is Bd6 according to the engine, and black is just down a queen, with white having an advantage of +5.46.