Tuscany, about 3 years ago. My husband and I were dating, and as poor college students, it was a big date-night splurge on what we were led to believe would be very good Italian food.
The overall feeling was akin to a previous poster's -- the experience was so astonishingly awful, I was left wondering if we weren't somehow caught in the middle of a prank. Nope, just a horrid dining experience. It began with an unexplained 20 minute wait: there were rows of empty tables, and yet the host made no move to seat us -- but was seating others who had come in after we had. We were finally seated, but with no menu. Our surly waiter brought them a few minutes later, then gave us water that, I'm not even joking, smelled and tasted like it had been wrung from the hide of a wet dog. Things continued to go downhill -- as I recall, there were zucchini flowers involved that, in terms of the greasy "ick factor" would have put McDonald's to shame; my husband's meat (steak, maybe pork - really don't remember) was frozen in the center, and Mr. Surly balked when my husband asked for it to be reheated; my pasta dish was undersauced to the point that I honestly thought they had just forgotten and had served me plain pasta. It was one thing after another, and by the time the check came, we were ready to run. Instead, after literally throwing the check at my husband, the world's angriest waiter simply didn't return to our table. After 10 minutes, my husband walked up to the host and handed him his credit card. We both came away from that experience with our minds absolutely blown.
In desperation, I once ate at an Olive Garden in Toledo, OH - it was too mundane in its badness to win any "worst-of" award.
The funniest bad dining experiences I've had have been with my friend Emily who, on three separate occasions in three very different restaurants, has not been served her food. Each time, she placed an order with the rest of us, yet didn't get anything. The first time this happened was in a Thai place on N. Clark, and, after it was clear that she wasn't getting her dinner, she called the waiter over to ask about her food. He stared at her blankly, repeated, "Food...?" and then hurried back to the kitchen. Periodically, we would catch him peeking around the corner, staring at her and, ostensibly, waiting for us to leave. It wasn't awful, just bizarre.