A project I’m on recently encountered a little known bug that I had to share, mostly so I can google my own brain later on this. MVC has a set of reserved words that cannot appear in your URLs. This includes CON, COM1-COM9, LPT1-LPT9, AUX, PRT, NUL, etc.

So how do I disable this lovely feature?

    <httpRuntime relaxedUrlToFileSystemMapping="true"/>

More details can be found at Phil Haack’s post about this same topic.