Forum Moderators: open
A few months on, after a google update, you'll find that all pages from your main site have been helpfully replaced with a one page entry pointing to your old domain. As an extra special bonus, you'll find that this one page entry has title, description and cache from the page you externally 404'd to.
</sarcasm>
Ah well, never mind. Hopefully i'll get a visit from damage-limitation-bot, um I mean freshbot ;)
ErrorDocument 404 [mainsite.com...]
There is a specific warning about using an external URL in the ErrorDocument directive documentation.
If a full URL is used, rather than a local path only, the 404 status code is lost but the custom 404 page is served in place of the requested page.
Use a 301 redirect rather than relying on a custom 404 handler!
RedirectPermanent / [mainsite.com...]
You have a few days before the next deep-crawl to fix this... I hope!
HTH!
Jim
I removed the the redirect completely and submitted the old domain for deletion via Googles urgent automatic URL removal system [services.google.com] which is explained on this page [google.com].
It looks like this did the trick because the old domain entry has been removed and my main site seems to be back in the index :)