Perhaps the most egregious thing they do is to use nonstandard grammar.

He often uses bad grammar when talking to suspects or people he does not respect.

I can't seem to use similar witty vocabulary and grammar for some reason.

It may use the same vocabulary and grammar, but in an unorthodox fashion.

The dangerous kind are all dirty and use bad grammar.

Some people use grammar that is different from other people when speaking.

"If you've got an English teacher who uses poor grammar, how can you blame students when they end up in my workshop 20 years later?"

Reading materials should use simple grammar and vocabulary so learners feel comfortable.

I use correct grammar and punctuation, not printing any of my letters in your paper.

Who would ever assume that the president's executive assistant wouldn't be neat or use proper grammar?