This one is secondhand from a coworker, so I apologize if I missed any details:
At first, it seemed like an ordinary enough transaction - just an Old Man and an Old Woman buying some groceries together.
That is, until the old man noticed a bottle of cold medicine by the impulse counter, and made the fatal mistake of picking it up and adding it to the cart. The old woman turned to him, her eyes narrowed.
OW: What is that, Harold?
The old man avoided her eyes, as she snatched the bottle out of his hands, and handed it to the cashier.
OW: Um, excuse me, but we aren't buying this.
She turned back to her husband.
OW: You don't need that useless garbage.
The old man stewed in angry silence for a moment, as the cashier continued scanning his items. It seemed like he was resigned to letting that particular battle go.
That is, until his eyes fell on the remaining item in the cart - a single carton of Neapolitan ice cream.
Then, a light flashed in his eyes, like the last ember of a dying flame.
OM: Ice cream? ICE CREAM?!
The old woman looked at him in shock, and opened her mouth to say something.
OM: No, you know what?! You don't DESERVE ice cream!
The old woman looked at him in shock, and as the cashier walked over to pick it up, (too shocked himself to really react properly) the old man ran over to him with supernatural speed, and slapped the carton out of his hands. The cashier tried to pick it up again, and the old man DID IT AGAIN.
OM: I JUST TOLD YOU WE'RE NOT BUYING IT!
To be probably overly generous, I think that OM thought that the cashier was picking it up with the intention of adding it to the transaction.
Still no excuse for that kind of behavior though.
The old man then grabbed his cart and receipt, and wheeled towards the door.
OM: Come on, we're leaving.
The old woman begrudgingly followed, the couple looking like a pair of quarreling siblings stuck in a get-along shirt.
Honestly, I feel like they kind of deserved each other.