My pet peeve is incorrectly punctuated signage, the most annoying subset of which involve abuse of quotation marks. A sign by the main entrance reads: ALL EMPLOYEES "MUST" SHOW I.D. All this does is make me not want to show my I.D. What, I "must" show my I.D.? But not really, right? Because it's in quotes?
Better hope they don't put up plaques reading PLEASE DO "NOT" CORRECT SIGNS WITH SHARPIE.