I think, it would have to be Ender's Game. Without revealing too much along the story, they missed so many key moments in Ender's "childhood" I ended up turning my TV off the first time I watched it. So what is your opinion?
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The Phantom Blood movie for Jojo's Bizarre Adventure, simply because it was so badly done, the author refused to let it have a home video version, and the only people that saw the movie were the ones that went to the premiere, I think