If you happen to run Ubuntu Server (at least, up to version 7.04) and use the Courier POP3 or IMAP server, and you're watching your log files closely, you've seen error messages such as the below one in /var/log/syslog:
May 21 01:14:10 server4 authdaemond: PAM [dlerror: /lib/security/pam_foreground.so: undefined symbol: pam_set_data]