P4Connect seems to always have trouble checking out the project files.
It seems to happen whenever the structure of the project changes (files are added, deleted or moved).
P4Connect - The following files could not be checked out:
/Users/juytven/Unity/Stealth/Assembly-CSharp.csproj
/Users/juytven/Unity/Stealth/Assembly-CSharp-vs.csproj
Further examination shows that the files are never added as part of the initial submit. They had to be added manually. Also a lot of shader files (created from rendering the scene) are also never added.