Yeah I'm sure there's been some, but its hard to think of any examples off of the top of my head. One I can think of is Haze, I was super pumped for a good 4 player splitscreen game plus it was early on in the PS3's life when I was desperate for good games...and of course it was shit, the demo plus all the horrid reviews turned me off.
Most of the time I just play demos for games that I figure will be write offs so most of the time the demo just confirms that it is indeed a write off. If its a game I know will be good I don't even bother with the demo really.







