I will occasionally think up witty remarks to say to snarky customers whilst I'm working, should I ever encounter any (and I do).
But usually what happens is something like this:
The customer, implementing some hyperbole: "About time! I've been sitting in the drive-thru for half an hour already!"
Me: "I'm sorry, sir. We're doing the best we can."
Customer: "Well it sure doesn't look like it."
He, your everyday megalomaniac, proceeds to be an unnecessarily difficult customer, often guffawing at the simplest of questions as if I just asked him to take his pants off.
But here's what, at the time, I wish I could have said:
The customer, implementing some hyperbole: "About time! I've been sitting in the drive-thru for half an ho-"
Me: "Sir, can you count?"
Customer, presumably startled and confused: "What kind of question is that?"
Me: "Well, if you could, then you would realize that there are a mere eight employees currently working, only three of which are working on the drive-thru. With it being night-time, when we're the most busy, I would go so far as to say that, presently, there are at least 50 customers, many of which have multiple items they're ordering. Doesn't it make just a little bit of sense to you that you had to wait for ten minutes?"
Customer: *Breaks down into tears, acknowledges that I am entirely right, and then proceeds to give me $100 as recompense for his ignorance.*
One can dream...