Agent P is surprised when he sees Phineas when he can't transform into pet mode and has to reveal his secret identity to his owners. Phineas deserves to be an agent at the O.W.C.A. Larry the Lemur makes a cameo in a deleted scene.
The creators discuss over how Phineas even could be an agent
At a Doofenshmirtz Evil Inc. station the jingle plays: Doofenshmirtz Evil in-
The creators notice they haven't put Dr. Doofenshmirtz there and say the episode is done
Ferb pops out of nowhere and said that, "they forgot nothing."
Note: It can be paired with either "Time Reverse" or "Phineas and Ferb: The Great Cliperoo"
It is in the third season