Google lists 404 error among the top five web page errors. 404 is the HTTP status code for “Page Not Found”.
It is caused when a server is unable to find the desired resource. The 404 error is very commonly confused with the server not found error. The difference between the two is that the former occurs when a server fails to locate a resource that a client has requested while the latter happens when the destination server cannot be connected. If the 404 error message shows up for a page it may mean that the requested content is not available at the moment, but could possibly be available later. This error indicates the temporary unavailability of a web page.
410 is another very similar error. If an error message for a request shows up as “Gone 404”, it means that the requested page/content has been intentionally removed permanently. Presently Google treats the 404 error and the 410 (Gone) error alike.
A 404 error shows up when a requested page is moved or deleted. In case you are moving a page to a new location, you can make use of the 301 redirect. If you wish to remove a page completely, you can make use of the 410 message. However you need to acquire special server configuration to make such messages available to the users.
Common Causes of the 404 error –
Moving a page
Websites often reorganize their pages or introduce new URL structures. In booth cases it is very essential to bring this change to the search engine’s notice. It is important to ensure that each such change in the URL must be accompanied with a 301 redirect to the current and functional web page. Links to these pages may have already been established on various sites as part of the off-page optimization activities. This makes the redirecting even more crucial.
Renaming a page
Renaming a web page must also be immediately followed with a 301 redirect.
A user may try to type in your URL. The type-in traffic is always likely to make spelling mistakes. You can try to acquire URLs which are very close to yours and if any potential type-in traffic is likely to confuse your URL with it.
Typographical errors are one of the greatest causes of the 404 errors.
Misspellings too result in the 404 errors.
Case sensitivity is never a problem when you are using only the top level domain. However if your URL contains the name of a particular directory or file, the case of the letters matter. Linux/Unix Operating Systems acknowledge the case while Windows does not differentiate between the lower and upper case. Case sensitivity also depends upon the web server setup. In case a webmaster decides to migrate from the current operating system, several technicalities need to be sorted first.
Many browsers expect favicons in a website. Whenever a browser tries to locate the favicons file for a web page and is unable to locate it, a 404 error is returned. To avoid a 404 error due to favicons, you must include a favicon.ico file in each directory.
There may be certain pages on your websites that you may not want the crawlers to visit. You have a provision for this in the robots.txt file. This file helps the search crawlers in identifying the URLs of the pages that you would like to exclude form the crawling process. If the search bots try to visit these pages, 404 errors occur.
404 to Start is a good plugin for managing the 404 error.