The worst experience I ever had at a restaurant was in CLearwater Beach, Florida, and I'm hesitant to put the blame on the restaurant fully.
I was there with my best friend at the time and two gay couples. These couples were both financially wealthy, so they were used to getting what they wanted when they wanted it.
We had a waitress who was probably 70 years old (or more). She was doing the best she could, and at one point I talked to her and found out she was working so she could pay off some overdue medical bills for her husband. She was really a sweet woman and doing the best she could.
Well, she was slow to bring drinks, slow to bring appetizers, slow to bring the dinners, etc. She wasn't wasting time or not doing her job, but she just moved slowly (I honestly think she had arthritis in her knees), and finally my (ex-)friend Buddy flew off the handle by her telling her how terrible her service had been and generally making a scene in front of the entire restaurant. I was so embarrassed I wanted to crawl under the table. He totally overreacted to her slow performance, and while we were leaving, I went to the manager to put in a good word for the woman. She got everyone's orders perfectly right; it just took a little time to get from the kitchen to the table, especially with that many people to serve. She was only able to carry a couple of plates on a tray at a time thus needing to make several trips to bring everyone's orders.
That was mid-week on our vacation with these folks, but it was the last meal my friend Hal and I ate with Buddy and his partner.