os/macos/ SingleUserMode

Hold Command-S when booting.

Create New Admin Account

Apparently Apple closed this loophole see here. But see here for more. Boot in single user mode (Command-S). Then at the command prompt type (the # comments don't need to be typed):

/sbin/fsck -fy # check drive for errors
/sbin/mount -uw /
rm /var/db/.AppleSetupDone

and the reboot. This is good for old macs.