I wanted to check-in a file, that was check-out by myself on an other pc.
You can revert those changes by using the tf.exe.
Open a command prompt AS ADMINISTRATOR
cd "C:\Program Files (x86)\Microsoft Visual Studio 11.0\Common7\IDE"
tf undo /workspace:"MYPC;MYDOMAIN\rlisdonk" /collection:http://220.127.116.11:8080/tfs/MyCollection $/MyProduct/Main/Source/MyProject/Staging.cs