>It do the following in the Command Window:

OPEN DATABASE mainDBC EXCLUSIVE

and no error is reported.
>- This even thoughI can see mainDBC in the databases dropdown in the VFP toolbar.
>
>The I do:

VALIDATE DATABASE

in the command window.
>The resultant output states:
>"Need EXCLUSIVE writeable access to rebuild structural index."
>
>Why doesn't the EXCLUSIVE clause on the OPEN DATABASE command give an error?
>
>cheers

it return an error for me.

check ADATABASES(),
OPEN DATABASES doesn't reopen a database

this is a bug for me, doesn't reopen is correct,
but an error must to be fired because VFP doesn't execute the command.