I'm sure we haven't had a customer service moaning thread on this particular board yet, and I like reading people's experiences, so I'll crack off with a good and bad example I've had recently.
Bad: Myself and my lady ordered some coffees this morning at 'Caffé Ritazza'. I wanted a latte, and she wanted a mocha. However the chap rung it up on the till as a mocha and an Americano, so she politely asked him to confirm that we were actually going to get a latté. He screwed up his face and said to her, really roughly 'What drinks do you want then? Tell me!', so she repeated herself. He then said 'A latte is the same price anyway!', so I said, not nearly loud enough 'But it's not the same drink, is it?'.
Completely bereft of charisma, the guy then went on to say 'That's £5 anyway' in the most unfriendly tone. I turned to the gf and said 'That's £5 anyway? What school of customer service do they teach that in?'. Then the guy comes over with the drinks in non-takeaway cups, at which point my gf reminds him that we asked for takeaway. This guy slams the cups down and turns away without a word. Cue waiting for another 10 minutes before actually getting the drinks we asked for, all the time getting a really shitty attitude from this guy. Then, after we left, it transpired that he HAD given me a fucking Americano after all.
It's only a coffee, but it's really really simple to be nice and polite to customers that really aren't being awkward, and just want what they asked for.
Good: We were in Giraffe (see bargains thread for 50% off voucher!) and I ordered a burger and asked for it well done. When it arrived and I bit into it, it was uncomfortably pink on the inside. Since I'm pretty paranoid about food poisoning, I point this out to a waiter who offers to replace it. I tell him it's ok as long as he assures me it's safe to eat, which he does. So I finish up the burger, which was nice enough, and we go to pay. Then the manager comes over, unprompted, and starts apologising for the fact my burger wasn't cooked as we'd asked and insisted on taking it off our bill. I found this amazing because I hadn't really complained, I was just checking it wasn't blatantly undercooked. Top marks to the manager because the burger was a good £8 or so and I would have happily paid for it.