Brendan Kowitz wrote a blog post about an interesting anomaly he noticed as Googlebot was crawling his site. Apparently, in March of 2006, Googlebot's User-Agent string was changed which triggered a bug in ASP.NET webservers. What this would mean is that Googlebot would not crawl the pages properly; instead, it would get 500 Internal Server Errors, thereby assuming that the pages were unavailable, even though they are.
A number of WebmasterWorld members were hit by this bug, and their rankings dropped because Googlebot would find 500 Internal Errors when spidering the pages. However, that seems to be fixed -- the only problem is that the rankings are not restored. (Google, can you fix this?)
Forum disucssion continues at WebmasterWorld.