If you are reading this, you are most likely having issues with Visual Studio Upgrading your project item *.csproj file when collaborating with other developers. Your solution is either not loading or causing merging issues when working with tools like GIT.
The reason is different ProjectTypesGuids between SharePoint 2010 and 2013 solutions. It may look like a 2013 project item but you should double check the values in these sections:
If you have this infamous GUID (SharePoint 14):
replace it with this one:
GUID (SharePoint 15)
The change above should get your project to load and compile but to avoid this compatibility issue you should install the Microsoft Office Developer Tools for Visual Studio 2012. You can get this pack using Microsoft’s Web Plataform Installer and then searching for ‘SharePoint’. Once this is installed VS should generate the right GUID