Apologies, this is an ill-formed observation, but I think merits further
investigation.

Yesterday I changed the settings on the SC help forum (at
http://learn.open.ac.uk/course/view.php?id=3985) to enable sharing.

Coincidentally everyone (me and the two editors sitting either side of me,
which I'm taking to be a representative sample) who had previously been
subscribed to the forum had their settings changed so they were unsubscribed.

If this is normal behaviour should there be a warning about it the perils of
changing settings on forums (or is there and I've just missed it?) - seems
potentially dangerous though.

The problem is when you enable sharing or disable sharing, it doesn't update the existing subscription records.