Grammar and Usage Peeves

As I was typing today, my fingers accidentally stuck an apostrophe in a plural word. I caught it instantly, breathing hard. It is one of my pet peeves. I'm starting to see it everywhere, as in, The Markowski's went to the store or Stock up on the advantage's of insurance. Aarrgh.

Apostrophes are used for contractions:

it is = it's
is not = isn't

Apostrophes are used for some possessives:

Mary's ball, The Fratellis' horses, the nation's president, the Markowskis' house or George Markow