If you use Subdivision Set [Ctrl 0] it's seems to not be possible to undo this action using shortcut [Ctrl Z]
I don't have enough reputation to comment, so I'll post this here...
Undo seems to work fine for me.
- New scene with standard cube
- Press Ctrl-2, then Ctrl-0, then Ctrl-3 and watch how the cube changes
- Pressing Ctrl-Z repeatedly goes through the stages of 3 subdivisions, then 0 (which is the cube again), then 2 subdivisions.
You may want to post a .blend file if your problem persists, or at least describe which kind of geometry you are having a problem with.