An error, spotted in this awesome article on a revolutionary new 3D-printing device:
To save your life, a surgeon will first insert a tube, and carefully guide it through the clog.
If you guessed that the error was the second comma, you'd be right. The rule is: don't use a comma in a compound predicate. Some people blithely believe you can put a comma just about anywhere because "a comma marks a pause," which is an odious—and often erroneous—intuition, given its dangerous fuzziness (as when people alter sentences because something "doesn't sound right").
SIMPLE PREDICATE: Johnny came in.
COMPOUND PREDICATE: Johnny came in and sat down.
The and signals the arrival of a second verb that is also linked to the sentence's subject. Both verbs operate with equal force; there's no need to introduce a comma, which inadvertently makes it looks as though you're trying to write a list of actions.
RIGHT: Johnny came in and sat down.
WRONG: Johnny came in, and sat down.
You might think the comma could somehow sneak in there to mark a dramatic pause, but if it's a dramatic pause you seek, go with an em dash or even an ellipsis:
Johnny came in—and stared in horror at the bloody corpse on the floor.
Johnny came in... and stared in horror at the bloody corpse on the floor.
Stylistically, both of these are superior to a comma. Now look at this:
Johnny came in, and he sat down.
The above isn't an example of a compound predicate. It's a compound sentence: two independent clauses joined, in this case, by a comma-and locution. Note that there are two subjects (Johnny and he), each with its corresponding verb (came and sat, respectively).
Let's stray a little bit further into comma-related murkiness. Normally, you use a semicolon to separate two independent clauses; mistakenly using a comma in such a situation results in the dreaded comma splice.
WRONG: Johnny came in, he sat down. (comma splice)
RIGHT: Johnny came in; he sat down.
Semicolons can also pop up when used as "supercommas" in lists where items in the list have commas embedded in them:
For the party, I'm inviting Jack, my boss; Tom, my best friend; Lucy, his wife; and Mary, her sister.
Were I to use nothing but commas, (1) there'd be a bit of grammatical awkwardness at the very end of the sentence, and (2) it would seem as though I were inviting twice as many people as I'm actually inviting to the party.
But sometimes a comma can be used instead of a semicolon to separate independent clauses, and this is the only such exception I can think of. It's a bit of a writerly trick:
We came, we saw, we kicked its ass!
—Bill Murray, "Ghostbusters" (1984)
Bill Murray's triumphant riff off Caesar's Veni, vidi, vici is an example of a technique called tricolon, in which a speaker speaks in threes ("of the people, by the people, and for the people"). Clauses in tricolon aren't bound by typical punctuation rules. What's more, because it lumps three independent clauses together with no coordinating conjunctions between them (and, but, or, etc.), Murray's utterance is also an example of asyndeton: a chain of related clauses with no conjunctions between the links of the chain.* Thus the clauses really do become mere items in a list, so using a comma is perfectly justifiable.
I could go on, but I think that's enough punctuation nonsense for today.
*From here: "Asyndeton uses no conjunctions and separates the terms of the list with commas. [It differs] from the conventional treatment of lists and series..."