Tonight, someone played

1 e4.  e5

2. Nf3 Nf6

3. Nxe5....Be7

It's not in any of the main lines of Petroff, so I assume it's a bad idea, but I can't see anything hideously wrong with it.  It develops a piece and prepares for castling. It doesn't seem great, but I also don't see any trap that is about to spring.  Is it a truly awful move, and I just don't see why, or is it a so so move, but just not as good as moving it further, like to d6.

well black lost a pawn.

Play 4.Nc3 and he is a pawn down for nothing.What more terrible you want to happen?

Ah yes, I see.  In the main line, black can capture the e pawn after forcing the knight retreat, whereas in this line, he can'tforce the knightuntil after the pawn is protected.

Thanks.