If Internet Explorer (IE) detects an error message from the web server less than 512 byte, it shows it’s own “friendly” error message. As a web developer you should turn off the default friendly error page setting on IE. It will help tremendously when debugging simple HTTP issues. For more complex issues I suggest the excellent free tool Fiddler).
To change the setting:
This is a simple HTTP 404 example, but it clearly states the problem.