Pages Redirecting to a 404 Error Only in Internet Explorer

This one had me stumped. The control panel for our CMS worked fine, just as it had been since, well since forever. The problem was that when visited with Internet Explorer users were now being presented with a brief flash of the login page followed by a 404 Not Found error.

After repeatedly pummeling my desk with my head I noticed that turning off Internet Explorer’s friendly HTTP errors allowed the control panel to be viewed as expected. So why was Internet Explorer in particular interpreting the server’s response as a 404 when it was clearly a 200? I already knew that the server wasn’t issuing a 404 response and the Live HTTP headers Firefox plugin confirmed this.

The HTTP headers displayed by ieHTTPHeaders (Thanks for the tip Lyallzord) told a different story though.

The problem was eventually traced to an incorrectly referenced pngfix behaviour pngbehaviour.htc in one of the control panel stylesheets. To be honest it was news to me that it even existed, but the path was incorrect following a restructuring of site assets hence the broken reference.

It seems that Internet Explorer on recieving a 404 response for referenced behaviour will helpfully 404 the entire document.

So that’s nice.