Perhaps you should step into his/her shoes and try to understand it from their point of view.

Have you ever worked fast-food or retail? Workers are treated like absolute garbage by almost every customer and have to take some serious abuse because "the customer is always right"

Sure, this guy was probably rude, but maybe that "vet" was an ongoing problem in that store. Maybe the cashier got screamed at all day long and took it out on the vet. Maybe that cashier has medical issues or dealing with a family crisis and thus, again, taking it out on people.

Yes, some cashiers are just rude. But you never know the full story.

Sorry to preach, but I tend to be on the defensive with customer service employees and try to give them the benefit of the doubt. I don't even have to be working to see the horrible things going on at businesses by customers (and because I've worked customer service, I also know the good and the bad going on behind the scenes and that's equally as stressful when management sucks)