I wholeheartedly agree with the idea that the customer isn't always right. Even if they're blatantly wrong though I never tell them that, and still act politely towards them. It's not even for my job's sake since there's no way they would have fired me, but just because I don't want my boss getting the flak for me not being able to handle a little ignorance every once in a while. My bosses always side with our employees at least, although they will try to appease the customer. We never get blamed for a disgruntled customer when it was just them being rude or demanding things that we just can't supply (like brands we don't carry or being able to mark down clearance prices or overhauling a bike on a busy saturday), and usually whichever manager was around for the incident will have some supportive or commiseration words to say afterwards. I gotta say, it really makes for a comfortable work environment, and it shows in our general customer service since we all feel at home at the shop.

As to getting sarcasm back for sarcasm, I was completely un-sarcastic in my reply of "you can tell by the apron." That's just my usual reply if someone asks if I work there, and I say it in a non-joking but still friendly voice. His reply was funny, but terribly inappropriate. The other girl who works with me has had her apron strings untied by some idiot guys who come by, like it's in their rights as a customer to undress the female sales people. That's way worse.