That proof is the movie adaptations of works by Alan Moore. For those of you who don't know who the heck that is, shame on you!
In short, Mr. Moore is a very influential, and truly gifted, storyteller, particularly in the realm of comics. We'll get to his titles in a moment. Just know that he's opinionated, and generally considered cranky (I recently heard him referred to as a brilliant douche), but you would be cranky too if you had been treated as poorly in the business as he has over the years.
As for his works, they are the proof. The movie adaptations include From Hell, League of Extraordinary Gentlemen, V for Vendetta and Watchmen. He also one of the men responsible for the creation of the character John Constantine, though the movie Constantine is not an adaptation of any of his writing.
Really, I don't even need to deal with the works collectively, since the nature of the proof is the same with all of them. So I'll just stick to what might be the worst of the adaptations: The League of Extraordinary Gentlemen.
The movie follows the exploits of a team of Victorian era literary characters under the leadership of Allan Quatarmain as they race to uncover the sinister plans of a villain known only as 'The Phantom'.
Do you remember seeing the trailer for this? I remember thinking 'Damn, that could kick ass.' Mina Harker, Captian Nemo, Jekyll/Hyde, an invisible man, Dorian Gray, and Tom Sawyer. Awesome.
Then I saw the movie in the theater. Well, I can't say I disliked it. It was a lark, I liked the actors, I liked the action and the effects. Hell, I even kind of liked the convoluted story. So, okay, I liked it. In and of itself, it was an okay movie, though incredibly flawed and disappointing.
But whether the movie was any good or not doesn't really matter. What matters is that it varies so wildly in characterizations from Alan Moore's source material that it begs the question why name it "The League of Extraordinary Gentlemen" at all?
Interestingly enough, the makers of the film were sued by a writer whose screenplay entitled "A Cast of Characters" was very similar. The producers settled, which was really insulting to Moore, whom they never defended after being attached effectively as a plagiarist.
The stupid thing is why they didn't just drop Moore, and his title altogether, and just go with "A Cast of Characters" since they mangled Moore's source material to nonrecognition. It probably would have cost less in legal fees. And, the filmmakers wouldn't have to worry about pissing off fans of the source material. Which comes to the point.
LXG was a popular comic, but not THAT popular. Any fan of the source could have told a producer "I know it sounds like fun, but I just don't see how you'll be able to do it." That isn't to say it couldn't be done, just that if the goal was to make a butt load of money, this wasn't the thing. Almost any reasonable person who read and liked the comic book could recognize that it was not suitable for mainstream film franchising.
So why? Okay, so I confess I don't have proof. I lied. But I'm fairly confident that the producers listened to the fanboys who were clamoring for anything LXG at the time "Oh, make it. It will kick ass. Just don't have Nina be the leader, I don't get that."
In the end, source material is only as good as the film makers ability to comprehend it. So here's my suggestion to studio executives. The next time a property is present to you, you read it and find yourself thinking you don't get it, but it has a solid core following, then pass. Just steal the premise and hire some hacks to develop a knock off. Even if it's modestly successful, in 20 year, when someone wants to do a gritty reboot, you'll be able get the fees without an entanglement with the creator and you wouldn't have to listen to the alienated laments of source materials fan base.
