Yes, I realize that was a pretty pathetic attempt at a humorous headline...
Commas in a Series
A conjunction goes between the last two items of the series.
While some authorities say that the comma before the conjunction is optional, leaving it out may cause confusion, so it is better to include it.
Words: Use commas to separate three or more words, phrases, or clauses.
Phrases: This morning I woke up, got dressed, brushed my teeth, and ate breakfast.
Clauses: In fact, the bus was full of people who got dressed, who brushed their teeth, and who ate breakfast this morning.
I see more comma errors than any other type of error, including misspellings. It's something we English teachers just haven't hammered hard enough, I suppose... either that, or people just don't care that much. Probably both. Nonetheless, spelling errors are fairly easy to recognize even when you're not the greatest speller; most people have no clue they're even reading a comma error, much less writing one themselves. [sigh]
Incorrect: The street was filled with angry protestors, shouting spectators and police. (Leaving out the last comma makes it look like the police were shouting, too.)Correct: The street was filled with angry protestors, shouting spectators, and police. (Makes it clearer.)