Grammar Lesson of the Day: Commas

I could probably write an entire book on the proper use of commas—where they're needed, where they're not needed, and how people tend to abuse them. Today, though, I'm just going to talk about one specific instance: commas used with and and but.

I've been seeing this sentence construction quite a bit lately:

"John wanted to start a rock band, but lacked the proper materials."

Uh-uh. As Bill Walsh of the Washington Post says in his book Lapsing Into a Comma (which I highly recommend, by the way), this is a run-on sentence with two subjects: John and but.

In a compound sentence like this, you either need to reintroduce the subject, introduce an entirely new subject, or omit the comma altogether.

Correct: "John wanted to start a rock band, but he lacked the proper materials." (We've restated the subject, so the sentence is now grammatically correct with the comma.)
Correct: "John wanted to start a rock band but lacked the proper materials." (By omitting the comma, John becomes the subject for the entire sentence.)
Correct: "John wanted to start a rock band, but Suzie hid his guitar." (Here, we've introduced an entirely new subject, which means the comma stays put.)

Like everything else in the English language, there are exceptions, but you should generally keep your subjects in mind when working with conjunctions.

Have you seen any common grammatical errors lately that drive you crazy?