Japanese shop staff make stupid mistakes a lot and it drives me nuts.
Proper humble Japanese would be "this is what will become used by you," ご利用になります。 But when serving food they just plug in a noun and it becomes something such as "this will become coffee." if it's going to become coffee, then what the fuck is it now? Am I allowed to drink it? There is a polite form, コーヒーでございます, which is actually understandable, and basically means "this is your coffee."