Misuse of apostrophes!!!!!!! I could write a book about that particular peeve of mine.
And subject/verb agreement. I cringe whenever I read something along the lines of, "Everyone has their own way of doing things." Everyone
is singular. One doesn't say "Everyone have
their own way of doing things." To be correct, one should say or write, "Everyone has his own way of doing things"; "Everyone has her own way of doing things"; or "Everyone has his/her [or his or her] own way of doing things."
One can use gender-neutral language and still be grammatically correct.
When editing my cow-irker Jewel's training material, I had to correct her subject/verb agreement errors in almost everything she wrote. She didn't understand why, even after she took the business English class that my boss and I taught.
Of course, Jewel thought I was a snobby Eastsider and was no fun, and one of her reasons for thinking so was because I chose to use correct grammar. (Most of the time, anyway.