control file errors

Hello there,

Once in awhile, as I am trying to run various versions of the Postgres DB
engine I get a message on startup indicating that my control file is not up
to snuff. Last time it happened with Postgres 9.1 on OpenIndiana 11.

So my questions are as follows:

1) Why does that generally happen?

2) When that does happen is there a way to re-init that control file
somehow?

Thanks.

Boris.