If an error is made, there are convenient options to fix the problem from the console, without requiring the install disc. The availability of some options depend on the situation.
One solution is to reboot into single user mode, then remount the drive using mount -u /. Other partitions required for start-up may have to manually be mounted. To make it easier, you can use the mountpoints in /etc/fstab.
SLiM will let you log in to the console line, by entering "console", depending on it's configuration file, as the login user name and the root password.
<ctrl><alt><f-key> will pull up a virtual console, where you can log in to fix configuration errors.
If there are still problems, replace /etc/ttys with ttys from the FreeBSD install cd, then edit it from there.
If problems occur with logging in through XDM, check /var/log/xdm.log or ~/.xsession-errors
% less /var/log/xdm.log % less ~/.xsession-errors
A firewall's settings such as PF's may delay a windowmanager starting up from the login manager, possibly through the Xserver. Test a permissive firewall setting, to see if this is the problem.
If problems occur with Xorg check /var/log/xorg.0.log. Use the commands less or grep to help you find potential problems.