Linux-PAM has the potential to seriously change the security of your system. You can choose to have no security or absolute security (no access permitted). In general, Linux-PAM errs towards the latter. Any number of configuration errors can dissable access to your system partially, or completely.
The most dramatic problem that is likely to be encountered when
configuring Linux-PAM is that of
deleting the configuration file(s):
/etc/pam.conf. This will lock you out of
your own system!
To recover, your best bet is to restore the system from a backup or boot the system into a rescue system and correct things from there.