Wish I'd taken a photo of this , but was in the Gallery of Modern Art in Glasgow today, downstairs in the Cafe/Library area, and there's a sign that says at the library entrance:
"P.C.'s not working" - and someone has crossed out the apostrophe and written "Nae apostrophes needed". Well, it could mean "P.C. is not working" (one P.C.) as well as "P.C.s (several) not working"?