What do you mean?
For articles, natives say sentences with wrong articles in them too. Seldomly because they don't know it (still happens), but because they change what they want to say mid-sentence. Cases are always a fight between the pendants and people who don't care.
Plain incorrect grammar will of course be noticeable, but grammar isn't everything.
And even if my word choice, register, or what have you is wrong, I don't understand the attitude. Because when they make mistakes in English I don't even comment.
When we moved to a German speaking country from the US some years ago, we didn't have that same experience at all.