1. Not using the Oxford comma
  2. Mistaking possessive for plural
    Apostrophes don't make words plural! This is a huge problem during holiday card season. Merry Christmas from the Gardners...not the Gardner's.
  3. It's whoa not woah
    Spelling not grammar, but it makes me MENTAL.
  4. Obvs any sort of there/their/they're and the like mistake
  5. People who don't understand the beauty of colons, semicolons, and em-dashes.
  6. An its/it's mistake is its own special hell
  7. Overuse of adjectives.
    Most of the time you just need the one word. Someone should EL James this...but she probably won't listen as she counts her $100 million. Barf.
  8. The misuse of "literally"
    Suggested by @ChrisK
  9. Unique instead of unusual
    Suggested by @jennienina