Movies can undergo changes from the moment they're greenlit, right up to the point when the final cut is about to hit the can. Studios often bring in last-minute alterations, from casting changes to extensive reshoots, usually with varying results.
There are numerous examples of these 11th-hour changes paying off. Would Back to the Future really have been the same movie if Eric Stolz had played Marty McFly instead of Michael J Fox? And surely nobody would have taken The Terminator seriously if they'd given Kyle Reese a cyborg canine sidekick as originally planned.
When a studio or director switches things up so late in the day, the outcome of these changes can go either way, and it's often the case that they ruin what would otherwise have been a great movie. Last year's Justice League is almost a good example of this. Warner Bros made multiple changes to the superhero team-up during post-production and the end result was a Frankenstein's monster of a movie, a mish-mash of conflicting ideas from two different directors, Zack Snyder and Joss Whedon.
That said, we'll never know whether Snyder's original cut would have been superior, but there are plenty of examples of films that would have been better off without their last-minute changes.
9. Superman II - Dropping Richard Donner
Richard Donner did a great job bringing Superman to the big screen in 1978, so it was no surprise that the studio retained his services for the sequel which began shooting the following year, but the relationship between the two parties would soon sour.
There are conflicting reports regarding whether Donner quit or was fired, but the result would have been the same regardless: he left Superman II with the project 75% completed, so the studio brought in Richard Lester to refilm most of it.
Unfortunately, Lester seemingly graduated from the same school of superhero filmmaking as Joel Schumacher as much of the goofball tomfoolery in the sequel's cinematic cut was his handiwork. That includes the slapstick shot of the ice cream hitting that guy in the face, and Supes throwing that cellophane S at Non.
Donner revealed to the world the version of Superman II that should have been in 2006 when he released his director's cut on DVD. It could easily have been called Superman II: Good Version, though, unfortunately, some of the movie was cobbled together from scrapped footage and it really shows.