Tonight, something happened to half-corrupt the password of my one and only one admin account. I could log in and wake from sleep, but the password could not be used to authenticate for any admin tasks. Not through GUI things like System Preferences, and not through sudo. I looked through the hints here, but didn't find anything outright.
I booted from the OS X CD and repaired the disk (and it found two files with illegal names) and repaired permissions. I also used the Reset Password utility to change the password. However, that didn't change anything. I also tried booting in single user mode to use passwd, but that didn't do anything at all (it didn't even ask me for a new password; it just exited).
However, another hint noted that if the master password is set (via the Security System Preferences panel), you can reset a password through the login screen by entering bad passwords a few times. After a few bad login attempts, OS X asked me for the master password, which I did know and it did accept. It then asked me for a new password, and then warned me that I'd get a new default keychain. I think this last part was an important aspect of what went wrong.
I then logged in, and everything worked again. However, I my old keychain was still in existence and working.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2005100720075612