Google indexed a page on a site of mine that was not linked from any other page, ever. No one has ever put a link to it, and the directory contents were not browsable. How could this happen? I thought crawlers have no way to include a page that is not linked.
closed as off topic by heavyd, Gnoupi, th3dude, quack quixote Apr 29 '10 at 18:13
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You didn't link to it, but how certain are you that no one else did? Someone sharing it on Facebook, Twitter, a public chat log, their personal site, etc. could do the trick.
If you don't want Google to index a page, block the Googlebot via robots.txt or some other technique. Relying solely on obscurity will bite you in the ass.