Google Webmaster Tools' "Fetch as googlebot" feature is useful in debugging these problems.
See http://www.mattcutts.com/blog/fetch-as-googlebot-tool-hacked...
See http://www.mattcutts.com/blog/fetch-as-googlebot-tool-hacked...
I accidentally still had it on from trying to read an article that some site was showing exclusively to the googlebot but paywalling for everyone else. If I hadn't, I wouldn't have noticed.