This happens all the time on Styleforum, and it drives me fucking crazy. It seems to occur most frequently with words that end in vowels, most likely because a lot of people were never taught (or didn't care to remember) how to pluralize such words. To wit, we'll see things like "potato's" instead of "potatoes." It also happens quite a bit with brand names or other proper nouns: "Grenson's," "Pimlico's," and so forth. Finally, I see it all the time when people don't know how to pluralize words that end with a "y": "dandy" becomes "dandy's" instead of the correct "dandies," for example.
But then again, I see people pluralizing just about anything with a misplaced apostrophe: "car's," "suit's," etc.
Fucking stop it, you morons.