Plus when they do happen, whoever is writing it could have either no knowledge of what goes on in whichever universe they are in and what the character capabilities are, or they just have a basic idea and that isn't really all that much better either. Also being biased could also come into play.
Registered: May 2007
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Freddy vs. Jason was done right IMO. You have to honor the tone and spirit of the characters without altering key events to have them clash. This would be a tough one but I'm sure someone could work something which could be satisfying.
Klingons vs. Stormtroopers, Vulcans against Wookiees and ultimately probably becoming allies, Khan matching wits with Sith Lords, etc. Win, win.