[fen ""]
1. e4 e5 2. Nf3 Nc6 3. d4 exd4 4. Bc4 Qf6
This move 4... Qf6 defends both the d4 and f7 pawns; now if 5. Bg5 to drive it away, 5... Qg6 and the bishop becomes a liability, preventing White from taking on d4; furthermore the e4 pawn comes under attack as well. How should White play from this position after 4... Qf6?
According to the 365chess database, 5. Bg5 yielded a 6-2-0 record for amateur players. There were no master games.
The pawn on e4 isn't too much of a liability as long as Black's King is in the middle of the board.
Here's a game where White is a FIDE(?) 2095 player. White's cute little combination on move 12 ought to tell Black he's in for a long day.
[fen ""]
1. e4 e5
2. Nf3 Nc6
3. d4 exd4
4. Bc4 Qf6
5. Bg5 Qg6
6. O-O f6
7. Bf4 d6
8. Nxd4 Bh3
9. Bg3 Nxd4
10. Qxd4 Bd7
11. Bxg8 Rxg8
12. Qd5 Qf7
13. Qxb7 Rd8
14. Nc3 Qc4
15. Qxa7 Rc8
16. Rfd1 Qc5
17. Qxc5 dxc5
18. Nd5 Bg4
19. f3 Be6
20. Nxc7+ Kf7
21. a4 c4
22. a5 Bc5+
23. Kf1 Rgd8
24. a6 Rxd1+
25. Rxd1 c3
26. b4 Bb6
27. Nxe6 Kxe6
28. Rd6+ Ke7
29. Rxb6 Ra8
30. Rb7+ Kd8
31. a7 Rxa7
32. Rxa7 g5
33. b5
6. 0-0 which pretty much brings Black's blunder to light.