Abusing service / customer-facing staff fits in to this as well and is at once particularly revealing and particularly damning.
Whenever a customer is rude to me I just remember that they're probably compensating for how terrible their own lives are. If it was actually an issue on my part then one of my coworkers would've told me by now. Makes it way easier to move on with my day.
While this dude didn't earn his money I am jealous that I didn't think of this first.