Umbraco 7 or 8 - How to fix - Cannot bind source content type to model type

If you have come to work today and found your Umbraco 7 or 8 website offline, it seems to be related to having code in the App_Code directory...

I'm not sure if this is a .NET Framework, Azure or Windows update which has broken it, but the quick fix is to move the code into a class library or out of App_Code and compile it.

I have only seen Umbraco Cloud sites affected right now, but that does not mean it's restricted to that platform.

The error is along the lines of:

Umbraco.Web.Mvc.ModelBindingException: Cannot bind source content type Umbraco.Web.PublishedModels.MODELNAME to model type Umbraco.Web.PublishedModels.MODELNAME.