1. No Oxford comma
    The serial comma, like all great commas before it, reflects the way we speak. And it makes things clear. And it just looks right. I will never understand how leaving it out is ever ok.
  2. Incorrect use of semi-colon
    Two complete sentences, people.
  3. Adding 's when all you want is to make something plural
    Holidays are a killer for me. "Happy Holidays from the Smith's!" Who is the Smith?? And don't leave me hanging - the Smith's what? Dog? Pet hamster? What does the Smith own that wants to wish me a happy holiday?
  4. Use of non-words
    This is less of a grammar thing and more of just a let's stick to real words thing, but stuff like "supposibly" gets me. I once read that you should never make fun of someone who mispronounces a word because it means that they've learned the word through the reading of books. I like that a lot. But if you take that a step further, then people who use non-words didn't learn them by reading. I guess they learned them by listening to other people who also didn't read them. Supposibly that's true.