Commas

Q. I was always taught that one needs to put a subject after a comma and conjunction so that it joins two independent clauses. For example: “Sara picked a flower from the garden, and she smelled it.” So, per the rule, if there is no “she” in the second part of the sentence, it shouldn’t have a comma: “Sara picked a flower from the garden and smelled it.” I’ve seen that many publications ignore this rule. I’m wondering if this isn’t a real rule, or perhaps I misunderstood it? Thanks in advance.

A. It’s a real rule, and you’ve got it right, but please see CMOS 6.28–29 for exceptions.