Saturday the 14th
Now that's a name I've not heard in a long time. It brings me back. I can smell the independent movie rental place, the one with the low ceilings and the wall of NES games. Simpler times.
Lawnmower Man (read the book first, then watch the movie, then go WTF did I just watch)
I'm not sure that one gets back around to being good. Nailed the WTF part though. As a corollary, you can do the same with The Mangler. I saw the movie first, thought it was unremarkable. Then I saw it in one of the King short story compilations and was curious to see what they changed. It's even shorter than Lawnmower Man, like two pages. And they made it into a whole damn movie.
Look Who's Talking Now
...wait what