Mac OS X 10.3 has a feature that allows you to require a username/password when waking the computer from sleep or deactivating a screen saver. Normally, you must either enter the username/password of the user whose account was active when the screensaver activated (or the computer went to sleep), or -- if Fast User Switching is enabled -- you can click the "Switch User" button to enter the name/password of another account (which logs that account in).
Reader Jon McGhie notes that under 10.3.3 (and possibly earlier versions), any user with an administrative account on the Mac in question can enter their own username/password -- without clicking the "Switch User" button -- to gain access to the account of the user whose account was active when the screensaver was activated or the computer went to sleep. Jon writes:
"While I appreciate the administrator has powerful access, and that this may be a normal function, it is not normally possible when browsing the '/Users' folder to access these files, so I think it is a bug."
We should note that this technically isn't a "bug" -- at least it doesn't appear to be to us. Many operating systems allow administrative users to log in to non-admin accounts; after all, there are valid "administrative" tasks that require such access. As for security, remember that an admin user can quite easily -- via Terminal or by activating the root account -- access the files for any user on the computer. If anything, this login issue simply illustrates the importance of restricting administrative access to only those users who really need it for administrative reasons.