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.
Absolutely this.
My Mum worked in retail massive floor lots of staff and random customers and found it scary at times with aggressive customers.
I did a summer late nights on a short staffed shift one where acting shift manager kept buggering off - I remember one evening racing for the male cook outside on fag break - as male customer was really screaming in other young waitress face and throwing threats at me– there was just me and her in building.