1973's The Three Musketeers and its quickly released sequel, The Four Musketeers: Milady's Revenge, used the (then) unusual tactic of filming both movies at the same time, significantly cutting down production time. Hoping to take advantage of the same scheme, Superman and Superman II went into simultaneous production.
As production costs mounted and it became clear that lightning might not strike twice (especially if it hadn't yet struck once), production on Superman II was stopped so producers could focus on the first one. Soon after that film proved to be a success, production started again on the sequel — sort of. There were a few difficulties, including Marlon Brando, who wanted more money and took the matter to court. Making matters worse, the director was swapped, putting Richard Lester in charge instead of Richard Donner.
Donner quickly scrapped some footage — about 75 percent of the film had already been shot — and essentially started over, making the movie his own. With a final budget around $54 million, most of that was certainly spent on possibly unnecessary reshoots to create the version we saw on screen. It's yet another instance of two last-minute decisions creating massive production expenses but ultimately resulting in a film — and even a sequel — that critics actually enjoyed, and made some money.
Still, fans clamored for years to see Donner's original vision for the caped crusader — and they got their wish when the "Donner Cut" of Superman II was officially released on DVD in 2006, proving once again that the smartest companies know how to turn conflict into profit.