getting access to a mac when you don't have the password or have forgotten it

I recently came across a mac where none of the known passwords worked, including the root account. 

If you boot from an external drive you can copy a known good plist file into the following folder

/var/db/dslocal/nodes/default/users/user.plist

You can copy the plist file from your own machine ( that way you know the password). Then just rename to match the user account you want access to.