Soft 404 errors
Some websites report a “not found” error by returning a standard web page with a “200 OK” response code, falsely reporting that the page loaded properly; this is known as a soft 404. The term “soft 404” was introduced in 2004 by Ziv Bar-Yossef et al.[11]
Soft 404s are problematic for automated methods of discovering whether a link is broken. Some search engines, like Yahoo and Google, use automated processes to detect soft 404s.[12] Soft 404s can occur as a result of configuration errors when using certain HTTP server software, for example with the Apache software, when an Error Document 404 (specified in a .htaccess file) is specified as an absolute path (e.g. http://example.com/error.html) rather than a relative path (/error.html).[13] This can also be done on purpose to force some browsers (like Internet Explorer) to display a customized 404 error message rather than replacing what is served with a browser-specific “friendly” error message (in Internet Explorer, this behavior is triggered when a 404 is served and the received HTML is shorter than a certain length, and can be manually disabled by the user).
There are also “soft 3XX” errors where content is returned with a status 200 but comes from a redirected page, such as when missing pages are redirected to the domain root/home page.