At WDW?? Never, ever, not once!!!! Our "worst" experiances have been average service (and not "bad" average service but "good" average service, if that makes any sense).
I've had great food and service at restaurants that have had bad reviews on both. Our favorite meal last year was at Fulton's. Not only was the food EXCELLANT but the server we had was fantastic. We also had good (or even great!) food/service at San Angel Inn, Kona's, Crystal Palace, Tony's Town Square, CRT (dinner), Chef Mickey's, Brown Derby (fantastic!!!!) and Marakeesh (sp? the Moroco sit down place). Even the counter service food was good, I really liked Pizzafari at AK, Casey's and Columbia House at MK, Backlot express at MGM, and the food court near Inoventions at Epcot. Room service at the Poly was also very good. All the places I just listed we ate at on our last trip in Dec. 2004.
I don't know if the people with bad reviews are just unlucky (which is possible, with 100's of restaurants and 1,000's of servers, someone is going to drop the ball at some point), or way too picky. I've never had even close to a bad experiance at WDW, either eating or in any other aspect of my trip. DFi has traveled all over the world and has eaten at some very elite and expensive places, and said the food at WDW is top notch. And this is a man that is VERY hard to please, and if something happens that he doesn't like, he'll say so.