There are some privilege related bugs in some versions of the Mac OS. Apple has a utility to clean up these issues. See
According to Apple..."Most users of Mac OS X have not intentionally modified privileges and simply need a utility to reset system privileges to their correct default values."
For Mac OS X 10.2 or later, open Disk Utility (/Applications/Utilities/).
Select your Mac OS X startup volume in the column on the left of the Disk Utility window,
then click the First Aid tab.
Click the Repair Disk Permissions button.