Attempted login gives "System error" failure message
Entering a valid password for a valid user resulted in a failed login. User only saw "System error" appear below the password prompt.
/var/log/messages, I noticed the message "Clock skew too great" which reminded me that I had logged in to Windows 2000 the night before (it's a dual boot machine that almost always boots to Linux). This was my first time to power it up after that, and I realized that the old Windows OS had probably messed with the system time. When I checked the time, I saw it was off by an hour (presumably some sort of DST / standard time issue). I changed the system clock to the correct hour, and the problem was solved. Naturally, this just increased my "love" of Daylight Savings Time....