Google Webmaster Tools to help webmasters to recognize potential issues before they become real problems. Some of the issues are relatively small (such as having duplicate titles and descriptions), other issues can be bigger (such as your website not being reachable). Here’re few examples that’ve been discussed in the Google Webmaster Help Groups:

1. Is your server treating Googlebot like a normal visitor?
2. Does your website have systematically broken links somewhere?
3. Is your website redirecting some users elsewhere?
4. Is your server unreachable when we try to crawl?
5. Has your website been hacked?

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