When you get the senior discount without you or the cashier even asking for it...
I had this happen about a week ago at a Dunkin' Donuts. I was buying two breakfast sandwiches and coffee for myself and my wife, and the young thing behind the counter puts the senior discount in, which I didn't notice until I read the receipt. I had conflicting feelings, as I was kinda annoyed that she perceived me to be that old, while happy to save $1.32.
My wife (who is 8 years my junior) thought it was hilarious...