False negatives (failure to find a page on another site because it wasn't configured yet) can end up cached for anonymous visitors

Not sure if this can be fixed but it should at least be documented.

To be clear, what happens is i visit:

newsite.org/path-that-only-exists-on-old-site

but i hadn't saved the correct passthrough redirect settings for the old site so i get the 'page not found' message on my new site. Even after i configure Not Found Passthrough correctly, i'll continue to see the 404 page at that path (newsite.org/path-that-only-exists-on-old-site), rather than being redirected to oldsite.org/path-that-only-exists-on-old-site, until a cache clear anyway.