Here are a few common Web error codes:
400 Bad Request
Server does not recognize the URL. Double check to see that you have typed it correctly.
401 Unauthorized User
This usually appears when you do not enter the correct ID or password to access a protected Internet site.
403 Forbidden Pages
You are not being allowed access, for whatever reason. If you have registered with a site that requires a password, and still get this message, check to see that you have correctly typed the password.
404 Not Found
It could be that the page you are searching for has been deleted, or its name has been changed or modified on the server. You can try deleting portions of the URL, starting at the end, and see what comes up.
415 Unsupported Media Type
The server will not accept the request, because the media type is not supported
definition from W3Schools.com
500 Internal Error
The server was unable to send the html document to you due to an internal (server software) error.
501 Not implemented
This error occurs when for example, you have pressed the submit button on a form. The server replies with this error message because it does not support the feature that you have requested. This is not a common error, but typically occurs when new features or forms are implemented.
502 Service Temporarily Overloaded
Too many people are trying to access the same site you are. You may also get the message Network connection refused by the server or too many connections. Try hitting the Reload or Refresh button; if that does not help, try the site again later.
503 Service Unavailable
The page is there but is not accessible at the moment. Hitting Reload or Refresh a few times may solve the problem; if not, try again later.
676 The phone line is busy (WindowsXP)
This is a dial-up error. Setup Windows to make connection using Broadband.
678 Timeout connecting to server (WindowsXP)
Verify cabling and modem are in sync.
691 Access was denied because the user name and/or password was invalid on the domain (WindowsXP)
Verify username and password in the PPPoE setup. In many cases, the username and password is mistyped. Retype your username and password. Be sure not to confuse a lowercase L with a capital I, or a zero with a capital O.
718 The connection timed out waiting for a valid response from the remote computer (WindowsXP)
Verify cabling and modem are in sync.
734 The PPP link control protocol was terminated (WindowsXP)
This often occurs when a cable is unplugged while connected. Reconnect this cable and reboot, if necessary.
769 The specified destination is not reachable (WindowsXP)
Network card is not setup correctly. Reinstall the drivers.
In many cases, the username and password is mistyped. Retype your username and password. Be sure not to confuse lowercase L's with capital I's and zero's with capital O's. This is most often the reason users receive the Error 691 message.
Remake Dial-Up Networking (DUN) connections
In some cases the dial-up networking connection is damaged and needs to be deleted and recreated with correct settings. Visit Creating Dial-Up Networking (DUN) Connections for more information and instructions.
In some cases the connectoid is damaged and needs to be deleted and remade with correct settings. Visit Creating Dial-Up Networking (DUN) Connections and follow the step-by-step instructions.
Failed DNS Lookup
This could mean that you have incorrectly typed the URL (try re-typing it with a "www" in front of it if you had not before), or that the server you are trying to reach is down or too busy. Check the URL and try again.
NNTP server error
This may appear when you are trying to sign on to a USENET newsgroup, and could indicate a number of different problems. The host server could be down or you may have incorrectly typed the URL.
Helper application not found or Viewer not found
This usually happens when you download a file, such as a RealAudio sound file and your browser does not recognize it because you do not have the necessary application to view it.