We have all probably heard film studios making ridiculous decisions and I think the biggest culprit is Warner Bros. for the first three examples.
Examples.
1. The Batman films.
These movies are meant to be dark and after
Batman Returns which was considered by them to be too dark, the studio decided the next one should be made more family friendly. Thus the slide began.
Warner Bros. must have said "I think we have done something very stupid."
When
Batman Begins came along, the series came back with its darkness. Clearly they learnt their lesson.
BATMAN IS SUPPOSED TO BE DARK!!
2. Superman IV
When the movie received poor reviews from its test screening, they cut down the running time. Unfortunately, they took most of the exposition with it.
3. The Avengers (1998)
-see above-
The reduced run time cut out major plot points. What was left were several questions without answers.
VILLAIN: I want to control the weather.
VIEWERS: Why?
VILLAIN:
cut response
How did Mrs Peel know the password for the phone booth's elevator?
3. DOOM
When producers make movies based on video games, classic TV shows or cartoons, they want to make a great and
faithful movie that the fans will love.
Until the studio gets involved. 
STUDIO: We don't approve of the monsters originating from Hell. That won't go down well with the religious groups. Do something else. Make them come from genetic manipulation.
PRODUCERS: But that will alienate the fans. If we stay true to the game, fans will come back again and again to see it. That's where the ticket sales will come from.
STUDIO: We don't care. Either create a new origin or we cancel production.