I'm thankful that in all my years of working in retail, I never had a gun pointed at me. Having said that, random people get so bent out of shape about the most ridiculous, inconsequential things and it can be close to impossible to predict who it's going to be or what is going to set them off.

Based on my experiences, if a customer is being indignant even after explaining that I was helping them out, I would apologize and remove the discount or ring the order up differently (if it was in my power to do so). More often than not, they calm down after they get what they want, even if it's to their own detriment. Granted, after calming down, a not insignificant number of these folks will realize their mistake and then they have to figure out a way to save face so they can ask for the discount to be reapplied.