"Have a good one."
I hear this almost every day- sometimes multiple times a day- and it always bothers me.
Usually it's when I get my soup at lunch.  I set the cup on the counter, the cashier rings me up, I say "Thank you," and they reply (without hesitation and with about as much enthusiasm as someone traveling to a wake), "Have a good one."
'Have a good one?'  Really?  Is that all you've got?
For me, this is a sure sign that customer service, as a vocation, is in trouble.  Service reps can't even muster a "Thank you for stopping by,"  or "Have a nice day."  No... these would be too personal.  Too committal. 
We can post our dirty laundry on social networks multiple times a day, citing whether we're single, in a relationship, feeling those "monthly" blues, hung-over, fearing an STD,  and can't bother to utter a personal greeting or thank you?
 
For me, I don't think it's about courtesy as much as it is insincerity.  Do they really care if I have a good one?  Do they care whether I have one at all?

Next time maybe I should respond, "I do.  I assure you."  Just to see what they say.  Nah.  Too personal.