The Mask II (lost footage/production material from unfinished "The Mask" sequel film; existence unconfirmed; mid-1990s)
The Mask was a box-office hit in 1994 and made a genuine star out of Jim Carrey. Getting an animated series and influencing further adaptations (including its original comic book), there were, of course, talks about a sequel, The Mask II. It would have once again featured Carrey, Amy Yasbeck, and be directed by Chuck Russell. This sequel would have come out just a couple of years later.
However, Jim Carrey made it known that he highly disliked playing the same character twice (as it wasn't "challenging" enough), and turned down a salary of 10 million dollars for reprising the role in this hypothetical The Mask II.
A sequel eventually came out in 2005 as Son Of The Mask, which didn't involve Carrey and was critically panned.
Despite this, there is still some possible evidence that footage of the original sequel project had been shot, or at least some media created. Late video game magazine Nintendo Power (issue 77) ran a contest in 1995 where the winner would visit the set of The Mask II and even be an extra. It is unlikely that such a contest would be published without at least some significant production being already achieved.
The winner, Nathan Runk, was contacted by Nintendo Power after Carrey declined the role. Runk was given the option to either receive a cash payout ($5,000USD) or waiting for the movie to be made in the future (so he could have the originally planned prize). He chose the money.
In addition, Runk was given a multitude of THQ games, a unique The Mask II crew jacket (that he, unfortunately, can't find anymore), and a phone call from Leslie Swan, a Nintendo localization manager and editor who voiced Princess Peach in Super Mario 64.
The final issue of Nintendo Power (#285) included an apology to the winner of the contest: "To whoever won that contest: sorry."