Well, I should really say the "missing comma" that cost a Maine Dairy $5,000,000 in back overtime pay to its drivers.
At issue was the wording in a state law that was supposed to specify what activities entitled an employee to overtime pay. The law exempted employers from paying overtime to workers who did the following:
"The canning, processing, preserving, freezing, drying, marketing, storing, packing for shipment or distribution of: (1) Agricultural produce; (2) Meat and fish products; and (3) Perishable foods."
The problem arises in the phrase ". . . packing for shipment or distribution of . . . " It is ambiguous: either "packing for shipment or distribution" is exempt, or both "packing for shipment" and "distribution" are exempt. The drivers did distribute the perishable food, but they did not pack it.
The drivers lost in federal court. The appeals court reversed, saying there should have been a comma between "shipment" and "distribution." With the comma, both activities – packing for shipment AND distribution – would have been covered by the exemption. Without the comma, the drivers did not fall within the exemption.
The Circuit Court sent the case back to the lower court, and in February 2018 it was settled, with the dairy agreeing to pay $5,000,000 to about 127 drivers.
This dispute demonstrates the usefulness of the serial comma (also called the "Oxford comma," as it has traditionally been used by the Oxford University Press). This refers to the last comma in a list, and it is often omitted. Many style manuals discourage its use, but I can't imagine why. Does it take too long to type? Does it take up too much space? In my opinion, clarity is far more important than "style."
It's true that in many cases, it can safely be omitted. "The flag is red, white and blue." No ambiguity there.
But how about this? "George found himself in the awkward position of being at a party with his ex-wife, a police officer and a drug addict." How many people is George confronting? One? Is his ex-wife a police officer and a drug addict? Or are there three -- the ex-wife, the officer, and the addict? The serial comma makes all the difference.
Don't let anybody tell you that punctuation isn't important!