I'm a big believer in second chances, mostly because I can think of at least one bad meal I've had at every single one of my favorite restaurants. For the vast majority of restaurants, there are just too many variables to control to be able to produce perfection day in and day out. Perhaps I'm naive, but I rarely think that one bad meal signals an overall decline in the restaurant.
Although this anecdote isn't perfectly applicable to the thread above (being concerned with a general decline in a restaurant), I'll share anyway:
A few years ago, when my boyfriend (now husband) and I were dating, and were largely unaware of the food options in Chicago, we frequently dined at a restaurant serving "southern" food. It wasn't that bad, although I was never blown away. One night, we went and within a few hours, I was dying - one of the worst cases of food poisoning I had ever had. A few months passed, and Unbeknowneth convinces me to try it again. We go back, have a nice meal, and the exact same thing happens - I'm in agony for the better part of 24 hours. I finally say to him, "That's it, I'm never, ever going back there. Period."
After a few more months, and a few more odd occurences of similar GI distress, I start thinking a little harder about it. I realize that the one common element that's linking these experiences is not, as I had previously thought, this restaurant, but rather oysters.
And that is how I discovered I had developed "adverse reactions" to oysters, and in the course of doing so, trashed this restaurant to everyone I knew.