Let's take a look at a few examples that we ran across in the Google Webmaster Help Groups:
Is your server treating Googlebot like a normal visitor?
While Googlebot tries to act like a normal user, some servers may get confused and react in strange ways. For example, although your server may work flawlessly most of the time, some servers running IIS may react with a server error (or some other action that is tied to a server error occurring) when visited by a user with Googlebot's user-agent. In the Webmaster Help Group, we've seen IIS servers return result code 500 (Server error) and result code 404 (File not found) in the "Web crawl" diagnostics section, as well as result code 302 when submitting Sitemap files. If your server is redirecting to an error page, you should make sure that we can crawl the error page and that it returns the proper result code. Once you've done that, we'll be able to show you these errors in Webmaster Tools as well. For more information about this issue and possible resolutions, please see http://todotnet.com/archive/0001/01/01/7472.aspx and http://www.kowitz.net/archive/2006/12/11/asp.net-2.0-mozilla-browser-detection-hole.aspx.
If your website is hosted on a Microsoft IIS server, also keep in mind that URLs are case-sensitive by definition (and that's how we treat them). This includes URLs in the robots.txt file, which is something that you should be careful with if your server is using URLs in a non-case-sensitive way. For example, "disallow: /paris" will block /paris but not /Paris.
Does your website have systematically broken links somewhere?
Modern content management systems (CMS) can make it easy to create issues that affect a large number of pages. Sometimes these issues are straightforward and visible when you view the pages; sometimes they're a bit harder to spot on your own. If an issue like this creates a large number of broken links, they will generally show up in the "Web crawl" diagnostics section in your Webmaster Tools account (provided those broken URLs return a proper 404 result code). In one recent case, a site had a small encoding issue in its RSS feed, resulting in over 60,000 bad URLs being found and listed in their Webmaster Tools account. As you can imagine, we would have preferred to spend time crawling content instead of these 404 errors :).
Is your website redirecting some users elsewhere?
For some websites, it can make sense to concentrate on a group of users in a certain geographic location. One method of doing that can be to redirect users located elsewhere to a different page. However, keep in mind that Googlebot might not be crawling from within your target area, so it might be redirected as well. This could mean that Googlebot will not be able to access your home page. If that happens, it's likely that Webmaster Tools will run into problems when it tries to confirm the verification code on your site, resulting in your site becoming unverified. This is not the only reason for a site becoming unverified, but if you notice this on a regular basis, it would be a good idea to investigate. On this subject, always make sure that Googlebot is treated the same way as other users from that location, otherwise that might be seen as cloaking.
Is your server unreachable when we try to crawl?
It can happen to the best of sites—servers can go down and firewalls can be overly protective. If that happens when Googlebot tries to access your site, we won't be able crawl the website and you might not even know that we tried. Luckily, we keep track of these issues and you can spot "Network unreachable" and "robots.txt unreachable" errors in your Webmaster Tools account when we can't reach your site.
Has your website been hacked?
Hackers sometimes add strange, off-topic hidden content and links to questionable pages. If it's hidden, you might not even notice it right away; but nonetheless, it can be a big problem. While the Message Center may be able to give you a warning about some kinds of hidden text, it's best if you also keep an eye out yourself. Google Webmaster Tools can show you keywords from your pages in the "What Googlebot sees" section, so you can often spot a hack there. If you see totally irrelevant keywords, it would be a good idea to investigate what's going on. If you find that your site actually was hacked, I'd recommend going through our blog post about things to do after being hacked.
There are a lot of issues that can be recognized with Webmaster Tools; these are just some of the more common ones that we've seen lately. Because it can be really difficult to recognize some of these problems, it's a great idea to check your Webmaster Tools account to make sure that you catch any issues before they become real problems. If you spot something that you absolutely can't pin down, why not post in the discussion group and ask the experts there for help?
Have you checked your site lately?
Link - from Google Webmaster Central Blog
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