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How does Google handle “not found” pages that don’t return a 404?

How does Google handle “not found” pages that don’t return a 404? - answered by Matt Cutts

Matt's answer:

We have a fun question today from Blind Five Year Old in San Francisco, California, who wants to know, “How does Google deal with Page Not Found pages that are returning a 200 response code instead of a 404? Is this a form of spam? Can Google determine this mismatch algorithmically?” Well, I’m not sure that I’d necessarily call it spam. Almost all of the time, it’s not done deliberately, because if it were, then people would return a 404 on purpose. We do have a name for this at Google. I’m not sure whether we’ve talked about it externally before, but we call them crypto 404s. Crypto from the Greek, meaning hidden. So a Crypto 404 is a page that looks like a 404 to a regular user, but to a search engine, it returns a 200 code. And there is actually a team in charge of, you know, trying to write algorithms to deal with those sorts of issues, including these crypto 404s. So most of the time, it works pretty well. You know, if you look for some phrase like page not found, you know, you can reasonably write an algorithm that says, oh, this return to 200 code or response code, but it still looks like a 404 page, treat it like a 404 page. But the problem is, people do a lot of really weird 404 page not found pages. So there’s definitely a few that don’t look like a regular page not found, and they’ll still return a 200 code. So I wouldn’t claim that we can handle that perfectly all the time. But most of the time, we do have relatively good algorithms in place to detect these so-called crypto 404s.

by Matt Cutts - Google's Head of Search Quality Team


Original video: