pp. Marking up a public sign to correct or point out a grammatical error or typo.
2010
On Wednesday, October 13, at the Decatur Library, writers Jeff Deck and Benjamin Herson will sign their new memoir The Great Typo Hunt: Two Friends Changing the World, One Correction at a Time. It's an English-action-packed memoir of public guerrilla proofreading that ultimately brought the authors in conflict against a misspeller with power: the U.S. federal government.
2010
Guerrilla proofreading does seem to be a popular hobby for some people. They can get quite agitated about apostroflies, invisible hyphens, and other such entities.
1996 (earliest)
LIST OF THE DAY: Some unusual findings sent in by you guerrilla proofreaders and grammar irregulars out there: