I have seen a few SQL Server 2005 Failover Clusters running into this issue. Recently, a KB Article was published explaining 2 methods (KB934749) to resolve this issue.
One scenario when you can run into this issue is when you have a SQL Server 2005 Failover Cluster patched with Service Pack 2 or higher and you add a new node to the Failover Cluster, you could run into the issue then.
Another scenario is that the SQL Server binaries on the one node got upgraded to a higher build but one of the member nodes was not upgraded due to some fatal error. But this is a very very rare scenario because SQL Server 2005 setup makes sure it patches all the member nodes where the SQL instance is not active before patching the node on which the SQL instance is installed.