ROCKY V. My dad introduced me to the films one day, and I watched them all one after another. My dad didn't bother to warn me of the suckage that was Rocky V. As I watched it for the first time, I desperately hoped it would get better. But it didn't. It only got worse.
Same thing with the third TMNT movie. God, even as a 10 year old, I knew that movie SUCKED. Biggest dissapointment of my childhood.
EDIT: I'll go ahead and add the Double Dragon movie to that list. I begged my dad to take me to see it in theaters, since I was such a huge fan of the games. Holy CRAP... if you've never seen it, please, watch it. I'm serious. It's one of those movies that are so bad, they're hilarious.