Every week or two, I go to this one place to buy a dozen bagels. They're not that near here, but they're the best bagels around, so I go anyway. A week ago, I went in at a different time than I usually do, and was helped by an employee I'd never seen before. After I left, I discovered that I only had 11 bagels instead of the usual 13. I was already having a pretty bad day, and that made it worse. I was annoyed at that and the rest of the world for the remainder of the day.
Today I went in, another strange time, another unfamiliar employee. (They have high turnover anyway.) I ordered my dozen, and while she was filling it, I mentioned, as politely and offhandedly as I could, to just make sure there were 13, because I only got 11 last time. I didn't want her to think I was blaming her; I just felt I had to mention it to someone there, so they knew. She said she'd be careful.