That
was a pretty gross departure, Snassek, but the all time worst would
have to be
Battlefield Earth -- I bought my first (used) copy in the middle 80's and it had a blurb on the cover announcing that it was "soon to be a major motion picture." I waited about twenty
years for that movie to come out. I
wanted to like it very badly. So badly that it took 'til 45 minutes into it before I could admit to myself just how bad it was.
All it shared with the book (which I actually quite liked) was the title, a few character names and a handful of plot points. Otherwise totally different.

I don't know
what they were thinking.
Extremely disappointing.