Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nxe5/3...Nxe4/4. Qe2/4...Nf6

4...Nf6??
This appears to protect the knight, but in reality it loses the queen. Far better is 4...Qe7, 4...d5, or 4...d6, which only lose a pawn. After 5.Nc6+, he loses his queen with a hopeless position. This is known as the Copycat trap and has ensnared countless amateurs.