I'm surprised no one's mentioned "the film that doesn't exist". You know, the one that begins with the letter "H" and ends in "lander 2". The one that destroyed all the continuity of the previous movie and set the percendent of Worst Film Series Ever.
I actually recall reading an interview with either Mike or Joel back in the early '90s, stating that they would like to have done their take on it. Too bad they never did.
BTW, I realize that my above description of that certain film may be too vague for some people, so, for their benefit, I'll describe it as "the film that begins with the letter 'H' and ends in horrible pain and torment for the viewer". Know what it is now?