Sometimes people ask me “Does Google make any distinction in scoring between Apache, IIS, or other web servers?” And I’m happy to say “Nope. You can use either web server and Google will rank the pages independently of the web server platform.”
But someone in AdSense mentioned an interesting case that they’d heard of. Apparently, doing url rewrites in ASP .NET 2 can sometimes generate an HTTP status code of 302 instead of a 200. This issue isn’t specific to Googlebot (it would impact any search engine bot). The best write-up I’ve seen is at http://communityserver.org/forums/536640/ShowThread.aspx. Looks like one of the first places noticing this was here (note: that post is French; an English translation is here).
It sounds as though if this issue (ASP.NET 2 + url rewriting generates a 302 instead of a 200) affects you, your site may drop out of most search engines. So how would you debug this? Fiddler is one handy tool for Windows. For Firefox, you might use the Live HTTP Headers extension to see the actual request your browser sent, and the raw reply from a web server.
I would also recommend Google’s Sitemaps tool as well. That team recently upgraded Sitemaps to show more details on errors that Googlebot saw when we tried to fetch pages from a site. The upgraded Sitemaps console also lets you download errors as a CSV file for debugging. I found out that I had a few urls with errors:
Clicking on the red oval above lets me download a file listing the problems that Googlebot had crawling my site, for example.
(Thanks for mentioning this, Antoine!)