Authors can break rules for the sake of style occasionally, think fragments or ending a sentence with a preposition, but it should be consistent and not cause undue stress on the reader.
Stress on the reader? Yes, anything that pulls the reader out of the story causes stress. Too much reading stress and the reader disconnects. Or at least this reader does.
But, even writers who fancy themselves grammar geeks need editors. It's easy to get in the zone, fingers flying across the keyboard, and accidentally put its instead of it's. Spell checkers can't help you, because both words are spelled correctly.
In past posts, I've not only pointed out my Top 10 Writing Pet Peeves (e.g., irregardless, lose/loose, they're/their/there) but also 5 Writing "Mistakes" I Like (e.g., sentence fragments, occasional adverbs).
“Ill-fitting grammar are like ill-fitting shoes. You can get used to it for a bit, but then one day your toes fall off and you can't walk to the bathroom.”
What's your grammar pet peeve? Will it kill a great story for you?