My 23-year-old co-worker (A) lives near a convenience store that they frequent near-daily. A is a smoker, and usually buys cigarettes here. She and her roommate visit the store as they do almost every day and try to buy cigarettes and a 6-pack of beer. The clerk looks at A, rolls her eyes, and picks up the 6-pack, saying, "I don't think so." She moves to put the beer back in the cooler.
A: Excuse me?
Clerk: I know you're not 21, don't even bother.
A: I have my ID with me.
Clerk: I don't need to see it. You're underage.
Now obviously, the bad_service here isn't being cautious about serving underage customers. But refusing to even check their ID because you assume they're too young and denying a sale because of it? Bad_Service.