Even before anyone used the name "Karen" to describe this behavior, it's more or less impossible to work in retail without seeing it.
And even if you haven't, there's a chance you've seen a customer who fits the description make a scene in a store at the some point. Even if they're not actually throwing a public tantrum, you can usually spot them by their tendency to make unreasonable demands of front-line employees and immediately try to escalate matters to the manager when they don't get their way.
But while these customers can be annoying to their fellow patrons, they're nightmarish to whatever employee is unlucky enough to be on the receiving end of their abuse. And one customer who was very aware of this found a clever way to do something about it.