I expect many of us have had experience working behind the counter, or still do. I certainly have.
I think the worse thing is a very angry customer - and one who is out of control aggressive, and making threats, is just so frightening.
Just on a personal level, in my short time of working at the checkout, as a young woman (about 18 - Saturday job in the 6th form) I had, maybe 3 or 4 very memorable, very angry customers - you know, not just a bit angry - but the ones who cause a scene - the sort who start yelling, and even threatening you. All were male.