When I recycled the old door jamb from the kitchen/downstairs hallway, I ran into a problem where the ends were too short to reach the floor. I couldn't install the doorway lower because I wanted the casings to line up at the top.
To fix this, I used the same kind of trick that they use when cutting trim to install hardwood (without removing everything).
Here's how bad it was. Nearly an inch short.
Now, if I had planned better, I could have just lengthened the jamb before it was installed, but it didn't look like it was going to be this bad, and I skipped this.
So I just cut a scrap piece of a decent size (1 1/2" tall) and cut the jamb like this:
Once the notch was added, the patch was nailed in (toe nailed from the jamb above), and with some creative putty and patching it won't be noticeable anymore.
The other side was also short, but only by 1/2" or less so I left it as-is, since the other bedroom door is also botched at the bottom and it's not too noticeable. I'll just call it old house "charm".