For most of my supermarket career, I worked for a chain that was rather high-end. Which was good for me, as that filters out a lot of weirdos who would rather be at Wal-Mart. So much of the clientele were nice, respectful, and polite. There weren't many people I remember being that bad, but like anything in retail you eventually find the unreasonable folks.
During a winter snow-storm, our trucks came from Atlanta, and at the time Atlanta was pretty much frozen-over. We had enough fruits and vegetables, but we ran out of the plastic bags people often use to separate them from the rest of their groceries. I explained this to an older gentleman and he said "that's what they all say!" and he ran off with a snooty attitude. One of those moments where you wish you could say what you were thinking in your head.