Humiliation Conga: Not only did Pie-Man give Comic Book Guy a well-deserved pie to the face, but it also made him a laughingstock and blew his chance of having tea (and possibly scoring a date) with Nichelle Nichols.
Hypocritical Humour: When Mr. Burns warned Homer of the probability of community service, Homer fearfully complained that he didn't want to help people, even though that's pretty much what he's been doing as Pie-Man.
The Lopsided Arm of the Law: As Pie-Man's exploits increase, the Springfield Police Department feels undermined (even if it's somewhat nonsensical, because as Lou points out Pie-Man is targeting Jerkasses, not crime), and so goes after the vigilante with spotlights, chopper support, multiple men in full SWAT gear, snipers... and a "shoot on sight" order.
Noodle Incident: Homer did assault someone before and is warned by Chief Wiggum that he'll go to jail if he tries it again.
Shrouded in Myth: When Homer exposes himself as Pie-Man, all of the other Springfieldians go on to argue that they had directly seen or heard about Pie-Man, saying wildly diverging descriptions (and none of which fit Homer). Lisa simply says that Pie-Man has ascended to become a "symbol", and Homer exposing himself can't kill that.