freejung - 6:41 pm on Aug 20, 2010 (gmt 0)
Oh, I remember what it was now. I put a canonical link at URL A that pointed to URL B, but URL B was 301 redirected to URL A. This created an infinite loop: A -> B, B -> A. It's obvious why that's a problem, and I have no idea what would happen if you created a structure like that with just 301 redirects, but I'm sure it would be ugly. However, what seemed odd to me is that Google didn't just ignore the canonical link, which was obviously wrong, and just canonicalize URL A. Instead Google removed the page from the index until I fixed it so that the canonical link pointed to URL A. Since then, there have been no problems that I've seen.