I still remember the time a few friends and I were in sainsbury in the Alcohol section looking at the beer when a store worker came up to us and asked for ID. Obviously we were miffed as to why he'd even asked for it when we hadnt attempted to purchased anything..
Anyway, we couldnt be assed to argue so all handed over our Id (We were 19). The store worker turned around and said "You cant get anything here, you're not 21".
We know we're not 21, we're 19..
Well you need to be 21 to buy alcohol here
No we dont, it's 18 to buy alcohol
(Points to a challenge 21 sign) No, you need to be 21
Go get your manager
Manager comes over, looks at us, looks at our ID, then apologises for his staff member and wishes us a good evening.
I cannot stand staff in shops/bars who either do not know the law or are unnessesarily over-zealous.