The worst experience in a restaurant that I can remember wasn't really the fault of the restaurant itself, but of a restaurant across the street.
Der Brucer and I were fond of a place called Love's The Rib Joint, in Lakewood, CA. Great place to get Texas beef ribs, and we'd always take home the bones for the dogs. We were in the habit of being seated in the bar/lounge area, because it was quieter than the main dining room.
This one night, we were seated at the middle table. An elderly couple were at the end table. We had just started our meal when a group arrived and took the corner table, behind me.
I have never heard such language in a public place before! This was a mixed group, men and women, and the women were just as foul-mouthed as the men! And they were being quite loud about it.
The elderly woman at the end table was absolutely devastated. She and her husband had not finished their meal, but it was clear that she could not stay in such a hostile environment. Der Brucer and I managed to eat some of our meal, but requested the doggy bags earlier than normal.
What made things worse was that the group at the corner table worked at a Mexican-themed restaurant across the street. A couple of them were still wearing their name tags, with the name of their establishment clear for all to read!
Amazingly, the staff at Love's didn't once ask them to modify their language.