When creating a robots.txt file, if you specify something for all bots (using a *) and then later specify something for a specific bot (like Googlebot) then search engines tend to ignore the broad rules and follow the ones you defined specifically for them. From Matt Cutts:
If there's a weak specification and a specific specification for Googlebot, we'll go with the one for Googlebot. If you include specific directions for Googlebot and also want Googlebot to obey the "generic" directives, you'd need to include allows/disallows from the generic section in the Googlebot section.
I believe most/all search engines interpret robots.txt this way--a more specific directive takes precedence over a weaker one.
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