Filesystem check on startup
Marcus,

Yes, certain things (bad reboot, disk marked dirty, not cleanly unmounted on shutdown etc) will tricker a fsck (disk check) on startup.

If you startup in verbose mode you can see this happening.


Shut down your Mac if it is on.
Press the power button to start the computer.
Immediately press and hold the Command (Apple) key and one of the following:

The "s" key for single-user mode.  (Command-S)
The "v" key for verbose mode.  (Command-V)



In addition, much like other unix systems there are scheduled daily, weekly, and monthly tasks. Using a tool like OnyX you can see these and force them to run.