This has happened numerous times, in numerous locations. Maybe it's a small thing, but it's ticking me off.
Either stop keeping the milk you use for coffee in the same fridge as the ones you give to the customers, or start labelling them "COFFEE MILK, DO NOT SELL", or somesuch. I'm sick of getting 1/2 to 3/4 empty milks, but not half as much as I'm sick of getting rude looks for asking for a new milk. I paid for a whole one, I want a whole one, I think that's fair.
Also, if I asked for a sausage biscuit with egg at drive-thru, and I got one with cheese, don't give me a "What's your problem?" look for coming inside and asking for the right order. What if I was lactose intolerant? What if your particular brand of fake cheese made me sick? It's not the sandwich I asked and paid for, I didn't do anything wrong, don't give me a look.