Everything seemed normal among the aisles.

No more cougars picking out wine, no slow-moving carnivores in the meat section, not one sign of problems from produce to bread.

Closing time at the supermarket is an orderly time. The leisurely pace of the afternoon is over. Night shoppers know what they came for. They stride directly for it and then make for the cash registers, just ahead of the nasally voice announcing the store will close in five minutes.

It’s bliss.

The con artists are out, but even they move with haste. The 19-year-old with a month’s worth of peach fuzz on his chin is trying one more time to buy a 12-pack of beer and a jug of Allen’s. At closing time, everyone is in a rush, he figures. Good time to slip one by the clerks who are just dying to get home to their girlfriends and boyfriends and computer games.

“Can I see your identification, please?”

A few perfunctory slaps at the pants pocket. A look of annoyance practiced in front of the mirror. Wouldn’t you know it? He left his wallet in his other pants. He’s got cash, though, and he’s bought booze here plenty of times.

The clerk isn’t buying it and peach fuzz is sent away. At least he has the decency to leave quickly, humiliated as he is. He’s no problem for the others in line behind him.

Neither is the older couple in front of me a few registers down. At least that was my early assessment as I carried my groceries — three basic items and one that was just mildly embarrassing — to the registers.

But I’m wrong about them. I’m so wrong, I question the degree I earned in Horrible Customer Psychology.

The man is wearing a grungy T-shirt and even grungier plaid shorts. He’s overseeing the bagging of his groceries, which I mistake as a sign of haste. But there is nothing hasty going on here. Mr. Grungy Shorts is supervising what starts to feel like a parlor trick. Bottled items are to go in plastic, he reminds the befuddled bagger. Boxed items — your cereals, oatmeals and uncooked pasta — are to be bagged in paper. Fruits and vegetables are another matter, and don’t even get the dude started on meats.

Meanwhile, his wife is in front of the register, handing over coupons and questioning the honesty of the clerk. She’s pretty sure the item he just rang up was 50 percent off, she says with narrow eyes, but she didn’t see it go through that way.

The clerk has to stop sliding items past the scanner so he can scan the inventory on a screen. Right there, he tells the woman politely. It went through as 50 percent off, see?

The woman agrees, but her eyes are still slits. She doesn’t trust this kid, who is probably 17 years old and just trying to earn enough to pay the iPhone bill. Crooks are everywhere, she knows. She knows because her husband has told her so.

This scene gets worse instead of better. Soon, the woman is questioning every item that moves over the scanner with its indifferent beep. There was a coupon for that one, she says. But where is my 30 cents off?

Things have become so halting and tense that a store manager has wandered over. Mr. Grungy Shorts has also become aware of problems at the register and he won’t have it.

“I’m going to be going over the receipt with a fine-tooth comb before we leave,” he mutters. “We’re going to make sure it’s right.”

The manager assures him it will be right. The clerk just gazes blankly at his boss, waiting for direction.

Then he’s sliding items over the scanner again. Three of them beep without drama, but on the fourth, the woman is sure once more that she was overcharged.

And now Mr. Grungy Shorts has noticed a few boxed items in plastic bags and that just won’t do. He’s complaining about that while his wife continues to grill the clerk.

You know this scene. Other people in other lines are starting to look over. Conversation has come to a halt. I hear one long weary sigh and look around a moment before realizing it was my own.

Other clerks wander over and quietly ask if there’s anything they can do to help. Smiles are pasted on their faces. These might be teenagers — just kids, really — but they do their jobs remarkably well. It doesn’t take many hours serving the public before the lesson takes hold.

People are horrible, being the most important lesson of them all. They are fickle and demanding and selfish and condescending. They are rude, unreasonable, stubborn and maddening. They are quite often wrong. But can you tell them that if you happen to be clerk or bagger or store manager?

No sir, you cannot. That the customer is always right is a rubric as valid now as it was the first time a caveman sold a pointy rock to a fellow hunter. You can never argue with a customer and nobody was arguing with Mr. Grungy Shorts and his narrow-eyed wife as the long, closing-time standoff continued. The unhappy couple just kept making their irrational demands, the store manager kept assuring them that this business would be conducted fairly.

I wanted to see how it came out. But as the situation approached a full half-hour, I was invited to another line where I was rung up and sent along within about two minutes.

Which was just fine. I could pretty much fill in the blanks and predict how it would come out. That degree in Bad Customer Psychology? I got mine over a couple years pumping gas. I made it those two years without killing anyone with their own dipstick, which is a marvel in itself. But I also made plenty of observations about bad customers, the most important being that 75 percent of them are just that — shallow, cruel-hearted bastards who try to compensate for their own shortcomings by mistreating those they view as underlings.

The remaining 25 percent are more troubling. These are the mentally unbalanced, men and women with a host of issues that make interacting with others very difficult. They are paranoid and often delusional. They believe they are being persecuted — by you, by me, by strangers everywhere.

The troublesome pair in line at closing time struck me as this variety of customer, Helen and Menelaus come to do battle at Shaw’s instead of Troy. It was them against the world, and on this night, the world was represented by a kid in a green smock and a chirping gadget with a fierce red eye. Tomorrow it will be City Hall, the newspaper or the Water District.

Probably right around the time you’re in there trying to pay a bill. Good luck, sucker.