I've never "went off" on a customer, but I have called them idiots and liars in the past. Sometimes I mock them or become a complete ass hole, but that only happens in the worst cases.
My favorite story though was when this woman came into the store to tell me I needed to fire one of my drivers because she almost rear-ended him. When she walked in the door, the driver had just told me a funny story or something and we were both laughing. She assumed we were laughing at her and just started yelling from get-go. We couldn't help but laugh at her. She called us a bunch of stupid kids and said a store shouldn't be run by a bunch of people under 18. At the time, I was 18, the driver in question was 26, and there was another guy in the store who was somewhere in his early 20's so we laughed some more. She asked us all our names stating that she'd get us all fired for the way we were acting. I was having fun with this by now, so I told her our names were all on our name tags so I didn't feel the need to state them. She looked at them and got some off the wall names (Zach was somehow Allen, for instance). She yelled at us one last time, we laughed, and then she left. But!... then she turned around, scowled at us from the parking lot, and literally started jumping up and down and screaming. At this point, we were laughing so hard we couldn't breathe. Her husband or whatever jumped out of the passenger side of their car and yelled something about killings us or something. They left then. Even better, though. She called in the morning to complain and the guy who was running the store wasn't exactly the most... patient guy in the world with these types of people. He basically stopped her in the middle of her rant and told her he had no interest in hearing it and that she should call back later that night when the manager got in then hung up on her.