Yesterday was National Punctuation Day.
The official website is http://www.nationalpunctuationday.com/ In addition to providing information on the day itself, the site gives usage information on each mark.
In the course of my day job—or when reading the work of aspiring writers—the most common punctuation mistakes I see are:
Commas: Not every pause in a sentence requires a comma.
Misplaced Apostrophe: Don't use an apostrophe when making something plural.
Maybe I'm the only literary weirdo with favorite punctuation marks; maybe I'm not. In any case, Happy Belated National Punctuation Day!