I wanted to get this out of my FFn profile but wanted it somewhere...
One of my biggest pet peeves with writing (and it's not just fanfic although that's where I see it most frequently since the writing is often un-edited) is the improper use of apostrophes for possessives and plurals.
Cullens = More than one Cullen. "All of the Cullens were gathered around the piano."
Cullens' = Possessive of plural Cullen. "We were at the Cullens' house."
Cullen's = Possessive of singular Cullen. "The book was Edward Cullen's."
If you pay attention to it, you'll see how frequently folks use "Cullen's" when they write something like: "We went to the Cullen's after dinner."