Visual Studio Installer Project - Unable to build project output group 'Content Files from (Your website) (Active)'

Often when building MSI Windows Installers from ASP.NET Web Projects within Visual Studio.NET you'll get this error:

Unable to build project output group 'Content Files from SOMEWEB (Active)'

And you'll have no idea why this happened, no descriptive error message, nada.

Turns out that this is the MSI builder's way of telling you that it can't physically find a Content File that is referenced in the Project (CSPROJ, or VBPROJ).

The trick is turn on 'Show All Files' and open every folder until you find one with the Yellow Yield Icon. This subtle tip is telling you that this file doesn't exist on disk, but it does exist in the Project. Right-Click on it and Exclude From Project.

When the MSI Installer builder can't find a file marked as Content it fails