This error message is seen when an earlier version of the server executable is run against 'db.*' files that have been upgraded to a later version. For example, when using a 2004.2 server against 2005.2 'db.*' files.
In 2006.1 and later versions the message is replaced with something more meaningful. You will see a message similar to the following:
Perforce server error:
Database is at upgrade level 18 past this server's level 13.