Waiter here. 

We usually only do reservations and we always ask if it's a special occasion ( birthday , anniversary, honeymoon etc). At 8:30pm walks in a party of 3 . No reservation but we squeeze them in. 

After barely paying attention to me , talking amongst themselves during my introduction one of the ladies mention it's her birthday and asks for a birthday cake. Yeah, that's not possible. We need prior knowledge of you coming and a cake costs extra (which she found unacceptable). 

During dessert we still sang and wished her happy birthday but she was still not satisfied and claimed we "ruined" her night out. 

Who are these people ? I get one delusional person but how does this make sense to 3 people ?