It's not the biggest thing in the world but you'd hope that everyone would know when milk has gone off. Especially if that person works in a cafe.
Apparently not, this very polite girl gave me the milk for our cups of tea, we poured the milk and found lots of lumps in the milk (that definitely means the milk is off, right? It also smelt funny). So took the milk back and politely asked for some fresh, non lumpy milk. Well she went from being very polite to accusing us of lying 
So I poured some of the milk into one of the cups and pointed out the lumps... I have apparently turned the milk lumpy - How the hell did I do that 
Long story short we got some fresh non lumpy milk after explaining the problem to someone else. Who, as it happens, also thinks that lumpy milk is off and should not be served to customers.
I really don't think we were being unreasonable and we definitely didn't deserve to be treated like that.