People misusing phase and faze! I see it all the time!
Incorrect: my toddler is going through a biting faze, but it doesn't phase me.
Correct: my toddler is going through a biting phase, but it doesn't faze me.
Apostrophes to pluralize upsets me most though. On Mother's Day I had a FB status wondering how many kittens would die due to all the "Happy Mothers Day to all the mom's" posts. Sure enough, my newsfeed was full of them by day's end, including several from TEACHERS.