A grocer’s apostrophe is the name for an apostrophe used on a plural, e.g., twelve egg’s.
This term is not a compliment, but a comment on tendency of grocers --- who make a lot of signs, day after day – to suffer from gaffes of various sorts
Computerization hasn’t eliminated all the problems. Here’s this pricing gaffe from Walmart, the world’s largest retailer:
(thanks to Steve Frenda for the picture)
(thanks to http://sesquiotic.wordpress.com/2012/02/08/groceries/ for grocer’s apostrophe)