The biggest mistake screenwriters make is writing the screenplay before getting the logline right, which inevitably results in the plot not working and the movie failing at the box office.
Getting the logline right is mostly about ensuring the idea on which the movie is based can work effectively as a 1st act plot point.
The Johnny Depp movie, Transcendence (budget $100M; box office $103M; CBR 1.03) failed at the box office because the idea on which the movie was based is not a viable 1st act plot point.
The idea is this:
A dying man’s mind is uploaded onto a computer and he becomes omniscient.
To work as a 1st act plot