If you work in retail, customer service, or the service industry, sooner or later, you're going to run into a confoundingly indignant and angry individual who is going to ruin your day—leaving you thinking about this interaction for years to come. It's just a matter of sheer statistics; talk to enough people in a day, and sooner or later, at some point, you're going to have a grown adult having a full tantrum in the center of your store like it's some kind of performative art piece. This Karen…