Everything worked fine the last time I gave my MSBuild presentation. There were no changes to my files, so imagine my surprise when I load things up to practice today only to be met by failures. 

"C:\Demos\Introducing MSBuild\AccountManager\example01.proj" (default target) (1) -> "C:\Demos\Introducing MSBuild\AccountManager\AccountManager.sln" (default target) (2) -> (ValidateSolutionConfiguration target) ->  C:\Demos\Introducing MSBuild\AccountManager\AccountManager.sln.cache(81,5): error MSB4126: The specified solution configuration "Debug|MCD" is invalid. Please specify a valid solution configuration using the Configuration and Platform properties (e.g. MSBuild.exe Solution.sln /p:Configuration=Debug /p:Platform="Any CPU") or leave those properties blank to use the default solution configuration.

The problem indicated by this error message is that the solution configuration is Debug|MCD. More specifically, this means that the platform equals MCD. I looked through all the solution files to find this mysterious MCD platform. I found nothing anywhere. Then I set the property through the msbuild commandline, and everything worked perfectly. The perplexing thing was that I set it to default in the project file if it was empty.

That was the key. It wasn't empty by the time the project file got ahold of it.

Environment variables are passed into MSBuild as properties. Keep this in mind if you're trying to figure out while your builds fail from certain machines due to errors being thrown from invalid property values. In fact, my problem came about because I purchased a new laptop from Gateway. I'm not sure what software they installed that set the environment variable, but it has the potential to wreak havoc on your build process if transitioning to a new build server.