20. An Early Script Had An Angry Witch In It
Considering how well-served the magic of the film is by the lack of explanation - aside from Phil just being a pretty bad guy - it's surprising to discover that the studio initially wanted there to be a reason for the time loop.
It would have had Phil cursed by a spurned ex, called Samantha. Eventually, director Harold Ramis agreed with screenwriter Danny Rubin that it was better to leave it out:
"While I was brainstorming I did consider various ways he got into the loop, but the arbitrariness of that enterprise convinced me that it didn't matter and when I thought of NOT having a device it gave the whole enterprise an existential flavor that I liked... I never wrote a draft with a cause. When Harold and the studio got involved we had lots of conversations about the cause, all of it painful to me but it seemed to be what the studio was pushing for. I think Harold wrote a draft where the cause had something to do with Phil crossing the wrong girl. They made me write a gypsy curse scene. But ultimately the studio came around and none of those things were shot."