a look at the output window, as suggested was not very helpful:
The solution still loaded, CI Build was fine, TFS and Octopus deploy happy.
The answer: the problem turned out to be in the .sln solution file, two sets of projects were listed:
The fix: at the root of the solution (in my case) in Visual Studio 2013, right click, open folder in file explorer.
Use an editor like Notepad++ to edit the solution, and look for the phrase:
chances are there’s more than one of these. One needs to go, find which contains the correct projects + correct number of projects. In my case I had 2 of these sections. One section had SccNumberOfProjects = 10, the other SccNumberOfProjects = 9. I needed only to delete the block with the incorrect number of proejcts, bounded by these:
GlobalSection(TeamFoundationVersionControl) and EndGlobalSection
save the updated solution file, then let Visual Studio reload the solution following this change and things should return to normal.