If you get TF270015: 'MSTest.exe' returned an unexpected exit code. Expected '0'; actual '1' when running a Team Build in TFS 2010 and can’t get it solved, you should try installing the QFE provided on http://archive.msdn.microsoft.com/KB983504
Even thoug the problem is not listed in the documentation of this QFE, is still seems to fixed it for me. Other guys seem to have made the same experience, as you can see in the discussions on this QFE.
