9. Not Planning Out The Sequel Trilogy - Star Wars
In fairness, you can point to many mistakes made by the Star Wars franchise as a whole - in the prequels alone, letting George Lucas have free reign, the excessive reliance on visual effects, and over-explaining aspects of the lore (namely the Force) that were better left ambiguous.
Yet the handling of Star Wars' sequel trilogy has left all but the most forgiving fans totally exasperated, as it quickly became painfully clear that producer Kathleen Kennedy had no clear vision or through-line.
The Force Awakens may have been too much a retread of A New Hope, but it least built a firm foundation from which new stories could be told.
The Last Jedi was conversely the boldest and most daring Star Wars film since The Empire Strikes Back, albeit one whose subversions heavily divided the fanbase.
Despite the ambition of Rian Johnson's script, it's clear that he took one look at The Force Awakens and decided to go in a violently different direction, only for The Rise of Skywalker to then unpick those changes and deliver more trite, over-sentimental fan-service.
Stepping back from the trilogy and looking at where they went, it's embarrassingly obvious that there was no wider roadmap for the stories or the characters within them.
Though having everything set rigidly in stone in advance isn't a good idea either, the fact that there wasn't a defined set of A-to-B-to-C beats before The Force Awakens began shooting is, frankly, ridiculous.
Considering that Disney spent $4 billion acquiring the Star Wars IP, that they ambled so clumsily into a make-it-up-as-you-go new trilogy will forever remain one of Hollywood's all-time headscratchers.