The following scenario should be familiar to any Guinness drinker. It’s the morning after the night before and you’re dying. Your head’s pounding, your eyelids are like lead and you expend what little energy you have making far-too-frequent trips to the bathroom. It’s not just a hangover, though. You’re suffering from the aftermath of A Bad Pint.

The existence of the Bad Pint is hotly denied by some people. These people are usually called “girlfriends” or “wives”. So you have a conversation that goes like this:

Wife: Jesus. You look terrible.
You: Yeah, I feel terrible too. Must have had a bad pint.
Wife: Oh, of course. The Bad Pint. Nothing to do with the way you drank your own body weight in booze.

Your wife or girlfriend is right – but so are you.

The Bad Pint really does exist, and it’s the reason why I stopped drinking Guinness when I moved to Glasgow. With lager, you can be pretty sure that you’ll get a safe pint no matter where you go. With Guinness, on the other hand, the quality varies widely – particularly if you drink in the sort of pubs where people don’t usually order Guinness, such as pubs whose customers are younger than eighty.

When you order a Guinness in a pub that doesn’t cater for many Guinness drinkers, there’s a 50% chance of disaster. That could be a contaminated pint that tastes distinctly of bleach, or whatever foul chemical pubs use to clean out their beer pipes, or it could be a pint that’s bad in some other way, such as a watery gloop instead of the creamy goodness you’d expect from a pint of Guinness.

If you’re sober and someone gives you a Bad Pint, you make a face and immediately take it back to the bar. However, when you’ve already slugged back a half-dozen pints, moved to a new pub and then been given a Bad Pint, you’re pissed enough to think “ach, I’ll drink it anyway.” The thought of fighting your way back to the bar, persuading a dead-eyed barman that the pint’s crap and then choosing something watery instead of Guinness is simply too horrible to contemplate. So you drink it, with no thought to the horrific consequences.

You’re right: the Bad Pint really does exist. But unfortunately, your wife or your girlfriend is right too. While the Bad Pint is a terrifying reality, the only reason it goes down your neck is because you’re completely and utterly pissed.