Both perspectives presents opportunity to explore human interactions. A very interesting question providing for perspective while offering reflection. I could comment on both and shall try to share. With being on the retailer side of the question my retail experience was with the automotive service industry.
The worst retail experience I had was with a customer came in screaming and yelling while was simply emotionally out of control. Let me share firstly the complaint was for squeaking fan belts. Why that caused such emotional trauma I have no idea. Sadly, the complaint was not even with my place of business that I represented as the manager.
Her receipt was for another place of business. However, she was beyond control and it required us to contact the authorities. Eventually she was arrested and taken to be assessed and observed for mental health cause.
Later her daughter and son came to get the car. They apologized while shared she was in recovery while offered little more information. We offered to assess the complaint discovering a loose belt causing the squeak. It was adjusted resolving the problem and they were happy. Of course we did it no charge as a courtesy.
On the other side of the fence the worse I can think of was I pulled a twenty five pound bag of sugar from the bottom shelf at a grocery store placing it in my cart. It leaked because it had a tear in it. Unfortunately I did not know that and left a trail through the store from the isle to the check out stand. An employee discovered the trail and went ballistic with me screaming billigerently. I was shocked, embarrassed, and felt shame. How dare I?
Obviously I understood the safety factor that it could cause an injury to someone by slipping on it. However, I did not tear the bag while did not know it was leaking. That was the conclusion of the store manager who became quickly involved. Another associate escorted the person upset with me to the back of the store. I still shop there and have seen that associate, however that associate does avoid me and I do avoid that associate.