If you have had to delete your user profile in Windows 8 to get around the known Internet Explorer 10 bug, you might have run into this error when trying to run an app from Visual Studio
Error 2 Another user has already installed an unpackaged version of this application. The current user cannot replace this with a packaged version. The conflicting package is 00392a51-cf7d-4fd5-bde7-be48dcef9ec2 and it was published by CN=sempf ToDoTodayApp
The reason this happens is the Package Name parameter of Metro apps. In the Package.appxmanifest you can see the GIUD that is used for the unique app identity.
<Identity Name="00392a51-cf7d-4fd5-bde7-be48dcef9ec2" Version="126.96.36.199" Publisher="CN=sempf" />
All I had to do to get around the problem was alter the GUID, and the program ran fine.