The lad leaves and goes around the back of the store to a lane, where he sits drinking, sharing it with a younger lad. The police happen upon the scene and ask the lad his age. At first he says 21, then admits to being 17, then 16. They ask where he got the drink and he points the finger.
The retailer, who could have denied it because they didn't bring the lad with them into the store, puts his hand up and says yes, I sold it. They took the CCTV evidence and later the retailer got a letter summoning him to court. He had had so far a clear record in his 20 years at the store, having been on trading standards courses and having regularly passed test purchasing exercises.
So why drag this first offence into court? And, more to the point, why drag it into court four times? Hearing after hearing was postponed because the police didn't show up - and the police statement was riddled with mistakes, saying the lad had been born in 1959 (making him 50) and that he had purchased Lambrini sparkling perry.
There was a satisfactory outcome in the end. By mid-May the case was dropped with insufficient evidence. The CCTV tape was returned to the retailer's local nick and the officers there admitted that the lad looked 21. "The police even said anyone can make a mistake," he adds.
The trader, who says he has learned "never to guess", will now be able to reclaim solicitor's fees that he reckons are about £500.
Well, you couldn't ask for a clearer message could you? Don't guess.
These are challenging times. Just make sure you are at the right end of the challenge.