At one time or another, you or your clients are going to receive one or many of these Apache error codes. While Apache does give a brief phrase indicating what each error represents, it does little more than that. Here is a rundown of the most common Apache error codes.
404 Not Found – This is perhaps the most common. It means that the URL was either typed incorrectly or the page that used to be there is no longer there. Either way, you will not find what you are looking for at that spot. If you have moved pages, it is a good idea to setup custom error documents to redirect users to the right place.
403 Forbidden – If you set permissions on a particular page or directory to exclude “others”, Apache will display the forbidden error.
500 Internal Server Error – This expression is probably the most unhelpful of them all because it could indicate a number of different problems. It is usually related to a script (CGI, PHP, etc) that is not configured correctly or does not have the right permissions set.
503 Service Unavailable – This could mean that there is a problem with the server, such as high CPU load. In some instances it could just be a temporary glitch.
There are other codes, such as 200 OK, which means the transaction succeeded or 307 Temporary Redirect, but you will not normally see them. With any errors, you can set custom error documents in .htaccess.