SEO Tip #108: How Does Google Handle Not Found Pages That Don’t Return A 404?
Is returning a 200 instead of a 404 considered spam?
Matt Cutts: I wouldn’t necessarily consider it spam. A lot of the time it’s not done on deliberately because if it were then people would return a 404. 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 response code. There is actually a team in charge of trying to write algorithms to deal with those sorts of issues including these crypto 404s.
Most of the time it works pretty well. If you look for some phrase like “page not found” you can reasonably write an algorithm that says, “This is returning a 200 status code or response code but it still looks like a 404 page so treat it like a 404 page.”
The problem is people do a lot of really weird 404 page not found pages. There are definitely a few that don’t look like a regular page not found and they will still return the 200 status 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 algorithm in place to detect these so called crypto 404s.