tedster - 12:40 am on Sep 7, 2011 (gmt 0)
The obvious "explanation" would be that the two domains share the same IP address (do they?) and something in the server set-up is technically incorrect. Alternately, there could be a technical bug on Google's end - they used to happen years ago with shared IP addresses.
If the sites are not on the same IP address, then I'm out of ideas except for a total Google bug. If that's what it is, Google usually fixes them pretty quickly. They seem to be related to corrupted areas when they move data around their servers.