In order to trigger a debug build, I added a Console project to my Yttrium VS solution and ran with debugging. When I run NUnit on the resulting Yttrium.Test.dll (in the build\Debug folder) 39 tests pass and 0 fail.
On the other hand, if I do a regular build and run the unit tests on build\Release\Yttrium.Test.dll there are 7 failures and 13 errors.