The cashier you were rude to on a bad day won’t remember your name. You’ll remember who you were that day.

Rudeness to a stranger costs them a bad five minutes and costs you something longer, because you are the only one keeping the record. They go home with a blurred memory of a difficult customer. You go home knowing which version of yourself turned up while you were tired, and that knowledge quietly adds up.

Bad days are not the problem. What you do during one is. Decide in advance that anybody serving you is out of range when your mood is poor, and if you slip anyway, go back and say so. It is awkward and it works.