Joel Shumocker's (or however you spell it) Batman and Robin. You know, the one where Mr. Freeze and Poison Ivy team up?
Never just watch whatever pops up on the movie channel after your mom falls asleep on her sick day, it will kill you.
Also, the Super Mario Brothers movie. My poor plumber childhood icons...ruined cinematically. Forever. And five bucks says that movie is why Princess Daisy doesn't get anymore mainstream roles in the games.
Eragon is also up there, even though I don't really like the books. And...well, yeah, most adaptations seem to suck...I only saw the trailer for City of Ember, and I can tell they already killed it because THEY SAY THE SPOILER OF THE BOOK IN THE FIRST SENTENCE OF THE TRAILER.
And they replace the secret ending with sewer monsters apparently. Whoop-de-doo.