Unable to check into TFS and Build with Continuous Integration '

You can receive that error as well if you try to check in a file that is in a project that is not mapped in your build definition.

Let's say that you have a file named FileA.cs that is in a project named ProjectA.csproj. If you do changes in FileA.cs and this file is included in your changes, you need to map ProjectA.csproj in the Get source step of the build definition.

To save others from the headache. The issue was that we had CI builds per project. A file from another project had snuck in as well. So I was checking in for 2 different projects on 2 different solutions (Which both go to the same TFS server). So make sure you only check in for that one project!