g1smd - 1:14 am on Nov 9, 2012 (gmt 0)
a 404 can't get passed if you provide a 301 or 302 status code and a 404 can't cause a redirect.
Requests for URL A return 404 status on site A as there is no such page.
You start a new site, site B.
After a while you redirect all requests for site A to site B.
Some time later, Google starts asking for URL A on site B. This happens because URL A on site A no longer returns 404, but instead returns a redirect to site B.