I was getting 404 errors for some long URLs on a website I'm developing. After a bit of digging I discovered that this occurs when the length of certain aspects of the URL exceed configurable limits in IIS7. In this case the maxQueryString attribute of the requestLimits property needed to be increased in the web.config

<system.webServer>
<security>
  <requestFiltering>
    <requestLimits maxQueryString="4096" maxAllowedContentLength="4096" maxUrl="8192" >
    </requestLimits>
  </requestFiltering>
</security>

This fixed the problem instantly on my development server but on the remote server I now get:

500 - Internal server error.

There is a problem with the resource you are looking for, and it cannot be displayed.

And that's all the information it gives me.