I feel most might think about things that looked good but sucked, where as I always think about this:

I would have to say Kung Pow: Enter the Fist. Based on the trailers and the promotional material, I thought it really was just an old kung fu film brought to a NA audience for the first time or something. I had no interest in it. If not for my brothers wanting to go and my parents forcing me to drive them, I would never have known how God damn hilarious it actually is.

To a lesser extent, the original Austin Powers was the same, but I think it really just took the fact it spawned from a series of SNL skits for granted. I had never seen those skits, and the trailers made it look more like a romantic comedy about a guy who was just infatuated with, but not actually from, the 60's. I don't even recall it showing Dr. Evil in the trailers I saw on TV for it. Just some of the interactions between Austin and Vanessa.