IE6/7 follows most requests with a request to "/404"

Question

We have been seeing a very large amount of 404's in our error log, specifically for the actual route "/404".

Confusingly enough, these seem to be caused by IE6 and IE7. We don't officially support these browsers, but it seems that after each request they make to the site, they follow it with a request to "/404". This doesn't seem to impact the application, and it only does this in our AJAX app, not the rest of the site. Whether or not it does this following a request seems to be random, but most of the time.

We have caching disabled in jQuery. We are using Backbone with Marionette. I can provide any other details that might be useful, if you ask below.

Does anyone know what might be causing this behaviour?

Answer 1

I managed to track this down to a 3rd party library we were using.

We were using a Filepicker widget to provide image uploading, and the Filepicker library will revert to a HashTransport communication setup for IE6 and IE7. This requires that the iframe open a location on the hosting server. They defaulted to the location '/404', hence causing a request to happen. This led to the issue I was seeing.