Content_ed: On December 6 of this year, Google released an article called "Tips for hosting providers and webmasters" on their Official Google Webmaster Central Blog, in which they wrote:
"Soft error pages. Some hosting providers show error pages using an HTTP 200 status code (meaning “Success”) instead of an HTTP error status code. For example, a “Page not found” error page could return HTTP 200 instead of 404, making it a soft 404 page; or a “Website temporarily unavailable” message might return a 200 instead of correctly returning a 503 HTTP status code. We try hard to detect soft error pages, but when our algorithms fail to detect a web host’s soft error pages, these pages may get indexed with the error content. This may cause ranking or cross-domain URL selection issues.
It’s easy to check the status code returned: simply check the HTTP headers the server returns using any one of a number of tools, such as Fetch as Googlebot. If an error page is returning HTTP 200, change the configuration to return the correct HTTP error status code. Also, keep an eye out for soft 404 reports in Webmaster Tools, on the Crawl errors page in the Diagnostics section."
and in another Google article, "Soft 404 Errors" in the Webmaster Tools Help section, Google states:
"Returning a code other than 404 or 410 for a non-existent page (or redirecting users to another page, such as the homepage, instead of returning a 404) can be problematic. Firstly, it tells search engines that there’s a real page at that URL. As a result, that URL may be crawled and its content indexed. Because of the time Googlebot spends on non-existent pages, your unique URLs may not be discovered as quickly or visited as frequently and your site’s crawl coverage may be impacted (also, you probably don’t want your site to rank well for the search query [File not found])"