(When a customer orders water at our restaurant, it shows up on their receipts as H2O.)

Customer: *loudly* “I think you’ve given me the wrong bill. You’ve charged me for H2O. I only had water!”

Me: *speechless*

Customer: “I want a refund. I’m not paying for something that I didn’t even have!”

(She carries on ranting for a couple of minutes until her friend points out to her in a surprisingly calm way that H2O is water.)

Customer: “Oh, is it?! I thought that was juice!” *slinks out looking embarrassed*

Related:
Have You Tried Dihydrogen Monoxide