A project says it is under source control when trying to be opened by Visual Studio, ending up in opening failure:
"The project 'OpenXML Power Tools' is under source control. An error occurred registering this project with source control. It is recommended that you do not make any changes to this project."
If this error is not fixed, another error appears and opening the project is not possible:
"One or more projects in the solution could not be loaded for the folloging reason(s):
The application for the project is not allowed.
These projects will be labeled as unavailable in Solution Explorer.
Expand the project node to show the reason the project could not be loaded."
Edit .csproj/.vbproj file end remove all <scc> occurrences.