When you log in to Mac OS X Server, the computer will first run a check against the user names in the local NetInfo database, regardless of the presence of that name in an external NetInfo server database. If it finds the user name in the local NetInfo database, it will stop.
The computer will then check the password entered against the password it has stored for the local user. If you try to use a different password that has been setup for the network NetInfo account, it will fail.
To work around this, create a local user account with a different user name. For example:
Network NetInfo entry:
Username: bob Password: sled
Local Netinfo Entry:
Username: localbob Password: sled