Express Lane Limit

-Twelve items is an estimate. The trapdoor only opens after I scan the sixteenth item.

In a large supermarket, there will be more than one checkout lane clearly designated "N Items or Fewer" (or "N Items or Less," which makes Grammar Nazis complain). This allows customers who don't have a lot to buy not to have to wait in line before those who do.

In fiction, mounting violence ensues when someone blatantly exceeds the limit, or tries to stay under by making up rules about what can be considered as a single item.

There's a variant where someone is only a little over the limit in a way that everyone finds acceptable, but wastes a lot of time fretting over the limit out of principle, trying to decide what they don't need and what should or shouldn't count as one item.

Examples of Express Lane Limit include:

Jokes

Some students in Cambridge, Massachusetts do their weekly run to the supermarket. After getting everything, they go to the cashier. The cashier guy asks: "And, what are you studying?" - "How do you know we're students?" The cashier guy points to the express lane sign and answers: "MIT students can't read, Harvard students can't count."

Literature

Frequently turns up in Dave Barry's writings. Rules about the Express Lane are presented as amendments to the U.S. Constitution in Dave Barry Slept Here and Dave Barry Hits Below The Beltway. Dave Barry in Cyberspace refers to the Supreme Court decision in the case of Mrs. Bernice A. Whackerdorfer v. A Bunch of Really Angry People Waiting in Line Behind Her.

Live-Action TV

In Doc Martin, Martin's secretary is late to work on her first day because she stopped by the supermarket to get supplies, and got into an argument about whether she was entitled to use the "six items or less" lane (she had 20 items, but claimed the important thing was she had fewer than six types of item).
An episode of the Canadian show Hiccups depicts Millie Upton getting frustrated at a woman ahead of her having more than the allotted number of items and using coupons.

Music

The opening line of Martina McBride's "Love's the Only House" deals with her being behind a woman who has 35 things in an express lane.

Newspaper Comics

In one Dilbert strip:

Cashier: This looks like a lot more than ten items, ma'am.
Woman: It doesn't matter. I'm old and you must do as I say.

- She suffers a Karmic Death (kinda) when her purse eats her while she is rummaging for coupons for all her items. Old Dilbert strips were weird.
One Grimmy strip had Mother Goose at a "10 items or less" line. The cashier asks Mother Goose to move to a different lane, claiming she has a bag of 10 apples, a carton of 12 eggs, a bunch of 39 grapes, and a box of 1500 Cheerios. She decides just to buy a quart of milk... but the cashier says there are 32 ounces in the quart.
A One Big Happy strip had a twist, where a man asked if he could cut ahead of Grandma because he only had one item. She said yes, and he proceeded to pay with a check, causing Grandma to scare him off by ramming him with her basket. The cashier says she's shocked, not because of what Grandma did, but because she didn't do it sooner.

Webcomics

Freefall blames calculators.
Sluggy Freelance this problem found itself a solution involving trained monkeys.

Western Animation

The Batman, when Joker goes Bat-mimic he Joker gasses people for whatever 'crimes' he notices. Littering (can hits the bin and bounces out), jaywalking, and 11 items at a ten or fewer checkout; tsk tsk.
In the alleged episode of The Simpsons where Principal Skinner is revealed to be a fake, Edna throws away some items from her cart to join Marge and Agnes in a line to talk about Skinner.
One episode of Chip 'n Dale Rescue Rangers had villainous mad scientist Professor Nimnul trying to get a colossal stockpile of prunes (long story) out through an express lane, with predictable results (apart from Nimnul's retort: "I've only got one item - prunes!").
An episode of Stressed Eric had three such lanes: "Less than ten items but more than four", "less than four items but more than one", and "one item or less" (which the titular character promptly use to checkout one potato.
In the Johnny Bravo episode "Johnny's Inferno", one of the evil deeds that a demon makes Johnny do is going (slightly) over the checkout lane limit. The cashier doesn't have a problem with this, though, much to his disappointment.
The variant is used in the Family Guy episode "Brian Sings and Swings".

Bruce: I definitely need a breath freshener. Ooh, but that's gonna give me 11 items.
Cashier: That's fine.
Bruce: No, no, no. Rules is rules. Let's see what I'm gonna put back. Okay, I need the Reynolds Wrap and the bathroom tissue. I could do without the Triscuits, but they sure are good. 7 Up's the whole reason I came down here in the first place. You know what, I'm not gonna need the V8, 'cause I can just get some tomato juice at the mini-mart down the street. It's a little more expensive, but that's okay; I like to help out a small business. I hope it's okay if I pay in pennies. [dumps a whole bag of pennies on the counter].