This one is nice and short.
My job involves me doing the newspaper returns, i count them, sort them into their various days and them bundle them together, tie them off and send them back to the publisher. As i was tieing off one bundle this jackass customer walks past me and says to me very sarcasticaly "Gee, that must take a lot of brain power to do that job"
I was too shocked to come up with a retort as he wandered off but for gods sake how bloody rude is that!!
After a while i thought of some good comebacks such as Got more than you do, or beats being on the dole.
Hope you get hit by a car you prick.