The fastest I've ever flip-flopped in that way was probably with the Tobey Maguire film, Wonder Boys. I absolutely loved it while watching it, immediately emailed two of my long-distance bros that they needed to see it, and then retracted my recommendation 24 hours later. It's been a very long time since I saw it, so I don't recall what I ended up disliking about it... I just remember being very embarrassed I'd raved so much about the thing just a day earlier. I now no longer rate/recommend anything without sleeping on it first.
My wife and I were huge Simpsons fans through the first six seasons. But there was this really terrible episode in Season 7 which broke the spell. It was like smelling salts. We both simultaneously realized that we no longer liked the show (it had already been getting bad- we just hadn't processed it until then), and I don't think either of us has watched a single episode of it since. And whenever I happen to see a clip from a recent show it usually makes my skin crawl so much it colors my memory of the seasons I liked.
Vanilla Ice! I remember my girlfriend and I jamming to Ice Ice Baby in the car and being stoked whenever the video came on. But of course that didn't last long. As a sad postscript, about a year or two after he broke, my brother and I visited my uncle in LA, who was a connected music industry executive at the time. He took us to the offices of all these studios and labels, and people would shower me and my brother in free CDs (with the little notch on the jewel box so you couldn't sell 'em). When were at the SBK Records office, they opened up this cabinet full of every CD in their catalog, and by brother and I both took one of everything but the Vanilla Ice CDs. The ladies who opened the cabinet were hovering and noticed we didn't go for the Ice discs. They both tried enticing us to take them, but we politely refused. They both seemed pretty crestfallen that they couldn't even unload these things for free on a couple of jerk kids from the Midwest.
I agree with lots of things/people mentioned by others in the thread, but those were mostly longer transitions.
Also, I'm two episodes deep in Season 6 of Dexter now, and I'm beginning to wonder if this show has always been bad or has just outstayed its welcome. I remember really enjoying it before, but watching it now almost seems like a chore... I probably should stop I guess.