People who don't know better than to bug me.
Seriously though,
Typos from people who should be checking things, like on restaurant menus and in digi ads. Just type it out in Word first and then paste it. Apostrophes particularly send me reeling.
Bad grammar as mentioned above. Another that bugs me: For all intensive purposes. Wrong. It's for all intents and purposes.
Random humming without a tune in mind. I did a layout about this last year. Goes for whistling too.
Cell phones in movie theaters. I can see your screen. And if you need to be checking it every 5 minutes, perhaps a movie is not where you are supposed to be.