My rule for students is basically, "If you have the option to be re-specify the thing you're talking about, do it." That's a solid rule for standardized tests, and usually applies to everyday writing (I'll caveat that this rule clashes with "the simplest answer is the right answer", so it is dependent on the actual sentence rather than an "all-or-nothing" rule).
Other common mistakes are those you hear about in middle-school (that's not a knock on anyone; rather, I say that to prove how long-ago it was that most of us ever reviewed common grammar and writing rules):
"Let's eat, Grandma!" vs. "Let's eat Grandma!"
Tying this back to GPT, I've read (and seen) folks write without any punctuation whatsoever. I can't speak to how GPT truly handles that, but if it's anything like "normal writing" and understanding, then punctuation is hugely important.