The two most common comma mistakes I see good writers make
As hard as it will be for many of my colleagues to believe, I actually know a couple of grammar rules and even advocate that people follow them.
I just finished a couple of weeks of peer editing and saw good writers making similar mistakes, the same mistakes I see good writers make a lot. The problem, I think, is that our brains know the rules but our fingers ignore them under the pressure of deadlines or when the muse is inspiring us with good content. But for what it is worth, here are the two comma errors I see most often (so if you are guilty of either or both of them, you are in good company).
Punctuating compound predicates as if they were compound sentences
A compound sentence has two complete clauses joined by a conjunction. A comma comes before the conjunction. Example: John wrote the user guide, and Mary edited the installation guide. Two subjects each with their own verb.
A compound predicate has only one subject but two verbs. No comma is used. Example: John wrote the user guide and edited the installation guide. If you absolutely long for a comma in the second example, you merely have to add a subject as in: John wrote the user guide, and he edited the installation guide.
Good writers usually make this mistake when the predicate elements are long, as in: John wrote the user guide that went with the latest version of the TurboPro Archiver, and edited the installation guide that went with the X-Level Wiki Mapper for Linux.
Long, but still a compound predicate and no comma.
Separating a two-item list with a comma
A two-item list does not take a comma. You would never write: You can choose option A, or option B. But as in the first error, good writers confuse themselves when choices get wordy: You can assign users to a category based on the permissions you have granted them in Section 10.4, or based on the profiles you establish in Section 14.6.
Long, but still a two-item list and no comma.
What is getting in the way in both examples is long sentences and the mythical comma rule of "use a comma when the reader needs to take a breath."
No such rule. No NEED for such a rule. None of us has written a technical document so compelling that the reader forgot to breathe. In my own case it's as if I breathe without even thinking about it. My lungs suddenly get hungry for air and wooosh; it just happens. I don't think I've ever heard of a situation where a reader was found blue and unconscious with those around him saying, "Oh my god! If only the technical writer had told him to breathe."
At any rate, be mindful that our fingers and this mythical rule are out to put spurious commas into the otherwise well-punctuated prose of even the most seasoned veterans among us. Keep a watchful eye out.