It's called the wayward apostrophe, the superfluous apostrophe or the errant apostrophe. I call it the #@%*&$@ apostrophe. It's the erroneous use of an apostrophe in common plural words and in other contexts.

You've seen it: a carved sign declaring The Smith's live in the home; a scrawled banana's for sale; a menu listing fresh prawn's. Some usage guides call this a greengrocer's apostrophe, as it shows up so often in the produce section (orange's, grape's, apple's).

Fowler's Modern English Usage says it once was proper to use an apostrophe to create a plural when a noun ended in a vowel. Since the mid-1800s, it says, grammarians have condemned this. "But it continues to appear, to the amusement of educated people, in signs and notices," it says.

"Superfluous apostrophes are a symptom of unedited prose and of the inexperienced writer," The Cambridge Guide to English Usage says. "As applicationsof the apostrophe begin to shrink, expert writers and editors are also less certain about its use."

Garner's Modern American Usage notes the wayward apostrophe often finds its way into the word says. Google News turns up plenty of articles with say's.

"The only possible cure is increased literacy," Garner's says.