I'm with SmashMonkey
Example from a sign in my local gym:

Pilate's Class's Wednesday's and Friday's

I don't know why it annoys me so much either.
Perhaps it's the ignorance of the writer who believes there's supposed to be an apostrophe in there somewhere, but they can't be arsed learning where and why, so they just plonk it before the 's' just in case.

If you're not sure where to put it, then ask a 6 year old.
(DougQuaid Get ya azz to moars..., Sun 11 Apr 2010, 2:27, 9 replies)

it's just ridiculous
and rather annoying, too
( Smash Monkey lowering the tone of the whole internet, Sun 11 Apr 2010, 2:59, closed)

Pilate's Class?
Does he teach you how to crucify people who can't use apostrophes properly?
( idonthaveafunnynickname Drink-Aware? Sure. I'm aware I want another drink., Sun 11 Apr 2010, 5:37, closed)

"If you're not sure where to put it, then ask a 6 year old."
Are we talking about an apostrophe, or something else?
( flake has nothing against your right leg, Sun 11 Apr 2010, 5:39, closed)

Ok,
that one didn't come out right...
(DougQuaid Get ya azz to moars..., Sun 11 Apr 2010, 6:51, closed)

As my wife says
If you are not sure dont use one at all, youll still be understood and wont look a fool.
(gearanach #wankwankwank, Sun 11 Apr 2010, 13:58, closed)

I beg to differ
I get wound up no end when people write

Im for "I am"
its for "it is"
were for "we are"

and so on.
(Vice Admiral Sir Charles Cockbrush KCB MBE DSO, who is finding it increasingly difficult to log into b3ta.com wants to duck your aunt and kick your puppy, Tue 13 Apr 2010, 16:55, closed)

Remember kids...
an apostrophe does not mean "Look out! There's an S coming!"
(Wallilay has been bored of this shit since, Sun 11 Apr 2010, 22:14, closed)

That one fills me with rage.
I feel like carrying around a huge black pen and writing corrections on all the signs I see.
(TheWonderfulWombat It came to me in a dream., Mon 12 Apr 2010, 2:46, closed)

of the many things that I miss about New Orleans...
...misplaced apostrophes are NOT one of them.

The all-time classic?

North Broad Street, half-way between Canal & Elysian Fields, on the right-hand side:

HIS' AND HER'S UNISEX HAIR SALON.
OPEN SATURDAY'S.

Wish I had a picture of that classic.
(AnntichristSCoulter, Mon 12 Apr 2010, 3:54, closed)