If a webserver return "HTTP/1.x 404 Document Not Found" (or any other HTTP error), MSIE will display it's built in error message (which we all are quite acquainted to), not the custom error page if its smaller than 513 bytes. When you access Web pages from Internet Explorer 5.x and Internet Explorer 6.x, the actual text of an HTTP 500 message that is sent to the browser may be masked by an Internet Explorer "friendly" error message. Although you can turn off this feature manually for each client, this article also provides several server-side workarounds to inhibit the display of "friendly" error messages. According to the HTTP 1.1 specification, Internet Information Services (IIS) 5.0 returns a status code of 500 for any response that is not handled by another 1xx, 2xx, 3xx, 4xx, or 5xx status code, such as "302 - Object Moved", or "404 - File not Found". Because IIS 4.0 does not strictly follow RFC 2616, it returns Active Server Pages (ASP)-based Microsoft Visual Basic Scripting Edition (VBScript) errors with a status code of 200. Read more here.
