When people use the word "of" in place of "have".
For example, "He could of killed me". When a person takes the time to write something, don't they ever stop to think what those words mean? I see this frequently on another forum in my recent job search.
Anyone can make a typo or use a word incorrectly once in a while, but constantly making the same mistakes annoys me.
I find that to be more of a spoken issue rather than in writing. More the words just getting slurred together. I'm probably guilty of this, as a friend of mine loves to tell me that I don't "annunciate".