If I'm treated like an idiot then that's exactly what they get, I go really slowly, painfully slowly at some points and telling me to 'Hurry the fuck up' means I will very slowly look up, blink a few times, look confused and then say "Ok" and then go back to doing exactly the same thing at the same speed. I find the longest way around tables and move with a similar speed to a sedated snail.
You can see the frustration as especially if the customer before them has been civil I have served them quickly and efficiently and now it's taking them 10 minutes to get a couple of pints and a G&T.
Hey, they decided I was an idiot and the customer is always right, so I'm just giving them what they want.
I am also someone who will ID someone who is being an absolute arse -
"You fucking serving love or what?"
"Not without ID I'm not"
"I'm 30 and I don't have any FFS"
"Pint of coke then?"
One group quite recently at an event started scoffing and laughing and said "Aren't bar maids supposed to be fit?"
To which I replied "Aren't customers supposed to be 18, no ID, no sale"
As well as refusing service on the grounds of people being drunk because they're being irrational, aggressive, lost inhibitions and have seriously lost track of time -
"I've been stood here half a fucking hour, all I want is a couple of pints, I'll come and serve myself in a minute" (it's always half an hour 😁) when waiting 5-10 minutes in a packed bar.
"I'm sorry Sir, you appear to be under the influence already because of your behaviour, I'm afraid I can't serve you any more alcohol"
I also have a certain amount of discretion over freebies, upgrades and discounts - the people who treat me like shit never get them, the decent people do, and it's even better when the arsehole sees you give something to someone else, demands the same and is told that was the last one (even if it wasn't).