In this event from what I remember, the audience would watch episodes of Phineas and Ferb as Dr. Doofenshmritz would be hidden throughout the episode. At the end of the episode, a counter would appear on screen revealing how many times Doofenshrmirtz appeared throughout the episode. As far as I can remember, it aired only twice. Once in the late 2000s and again in the early 2010s. The episodes I remember that were part of the special were "Phineas and Ferb Get Busted", "Chez Platypus", "Come Home Perry", "That Sinking Feeling" and "Don't Even Blink". However all the episodes had "Doof" in the title as a play on words. That's about all I can remember. There is absolutely no footage of the episode apart from one small clip. If you have any information about this special let me know.
found this video linked by a comment in the spot the diff video: youtu.be/gKxFceAPJuM its a small portion of an episode with four doofs. i dont remember spot the doof so i dont know if the rest of this episode still needs to be found.