Legal's Trap also known as Legal's Mate often arises in shown or similar positions. In this example it works like an opening trap for Philidor's Defense. White pseudo-sacrifices the queen and mates by 3 minor pieces.
The trap is named after Sire de Légal (1702-1792) who was a French player, or Joseph Henry Blackburne (1841-1924), who was a British master and one of the world's strongest players in the latter part of the 19th century. Main Line:

1. e4 e5 2. Nf3 d6 3. Bc4 Nc6 4. Nc3 Bg4 5. h3 Bh5 6. Nxe5 Bxd1 7. Bxf7+ Ke7 8. Nd5#

When the Legal's Trap doesn't work?

1. e4 e5 2. Nf3 d6 3. Nc3 Nc6 4. Bc4 Bg4

The Legal's trap does NOT work in this position. The reason is black knight on c6. Now, if white plays
5.Nxe5? Black still cannot capture the queen (or the same mate as shown above would be developed), but instead black can take the knight 5...Nxe5 protecting the bishop at the same time. White loses material.