Do proficient writers make grammatical mistakes?
Oh yes, we most certainly do.
In my first draft, I usually make mistakes.
Even as I write, I am aware I am making a mistake; but carry on , so that the flow is uninterrupted.
I think yes they also make mistakes which are grammatically incorrect.
But over a period of time writers tend to improve a lot and they would be in a position to consciously avoid common mistakes they commits usually while writing.
Generally after finishing your work sometimes proof readers goes through the written matter diligently and spot the mistakes and corrects it before publishing.
But proficient or well known writers are known for thier skills in writing , creating wonderful stories or thought provoking ideas which makes them well known.
But mistakes happen which gets corrected after repeated reading by proof readers or by one self.
All the time.
Sometimes we make them on purpose.
Sometimes we read what we've written and do dog-head-tilt, and think "wow, whatever I was on, it must have been good stuff.
As both Donna and Al said, that's why we're writers, and writers are not editors and proofreaders, because while I try to check myself as I go, I'm almost guaranteed to make mistakes here and there.
If I don't catch them — despite my best efforts — someone else will.