Here is a pretty basic guide to the most common errors codes (most of which this site has seen) and how to fix some of them.
400 (Bad Request) - This usually means there is something wrong with the URL or the browser had some kind of error when asking to view files. Check the URL and make sure it is valid and not too long.
401 (Unauthorized) - This one comes up when you enter the wrong password in protected login boxes. Unless you know the username and password, you're not getting in.
403 (Forbidden) - You don't have access to view a file. This usually happens when looking in a folder that doesn't have a default index file.
404 (Not Found) - The file doesn't exist.
405 (Method Not Allowed) - This one occurs when one page sends data in the wrong way to another page.
410 (Gone) - Pretty much the same as 404. This shows a file used to exist.
414 (Request URI Too Long) - The URL is too long. Try shortening it.
417 (Expectation Failed) - The header of the page request has an error in it. This one occurs when a page doesn't send the right information to another.
450 (Blocked by Windows Parental Controls) - WPC doesn't allow access to the site. To fix it, add the site to the exception list.
500 (Internal Server Error) - Something happened on the server but it cannot say specifically what. Sometimes, a simple refresh will fix it.
503 (Service Unavailable) - The server is overloaded and either has too many requests or is running too much at once. Refreshing might fix it.
509 (Bandwidth Exceeded) - The site has used more bandwidth in a given month than allowed by the host.
Of course, there are many more errors than these, but these are the most common ones.
