My mother and I went to eat at a restaurant that was part of a seafood chain (which no longer exists I'm afraid), I was a vegetarian at the time. Mom ordered fish, I got the steamed vegetables.

The food was delivered and we dug into our meals. I looked down to start on my broccoli when I saw a worm darting around the broccoli and hiding underneath one of the stalks. My mother saw it and we just stared until I started giggling.

I called the waiter over and said "I can't eat this, I'm a vegetarian" and lifted the broccoli. The worm made a break for it. The waiter screamed.

He grabbed the plate and RAN with it to the kitchen. My mother and I were killing ourselves laughing when the manager came over to apologize and offered to get me anything I wanted to eat, on the house. I had pretty much lost my appetite, so I thanked him and said "no thanks." We didn't have to pay for that dinner.

But you said worst.

I was with my boss and coworkers at a large, well-known italian restaurant chain for a lunch to celebrate the completion of this project we had worked on. The food was delivered, my spaghetti looked like it was scraped from the bottom of the pan. The woman sitting on the side of the table where traffic was moving behind her got bumped by a waitress, whose tray tipped and spilled a very large glass of iced tea all over her head, her suit and all in her purse. She didn't get an apology, all they said was "it was an accident."

She asked for the manager and insisted the restaurant pay for the dry cleaning of her suit, he just pleaded "accident." The whole staff was indignant that she was raising a fuss over an "accident" - I didn't even bring up my shitty spaghetti, which was mostly left on my plate.

It was YEARS before I ate at one of their restaurants again.

EDIT:

Then there was the time my friend (who was in a wheelchair) and I were spending the weekend in Niagara Falls. We went to that rotating restaurant which was NOT busy at the time, and they seated us. We waited a half hour. Groups came in and were eating their food and we didn't even have menus.

We asked the waitress three times for a menu, each time she's telling us she'll be right with us. We're watching people that came in 45 minutes after us eating their food and there we sat. We decided to eat somewhere else, so we were waiting for the elevator and one of the staff said that the waitress would be right with us. I told him that we were told that more than an hour ago.

We went to the diner across the street and the service was equally horrible, but at least we got our food. We're waiting for the waitress to bring the check so we could leave (we were ready to drive home and it was now almost noon before we had our breakfast). I asked if there was a waitress around so we could pay our bill. She did come over and practically threw it on the table. I got up to pay the bill at the counter as my friend is wheeling herself to the car. I passed the table on the way out and was furious to see she left a tip for the waitress. I beat her to the table, looked her in the eye, grabbed the tip and walked out.

Then there was this swanky place where the service was so bad a friend of mine left a 30 cent tip - all in pennies, sticking out of the leftover mashed potatoes, in the water glass with the tampon (thankfully not used).