It was a test update and there will undoubtedly be a review of this. http://www.zdnet.com/article/microsoft-accidentally-issued-a...

The fact that a test patch got to this stage doesn't mean the safeguards aren't in place or that malicious code could have slipped through, though. Assuming even basic competence, this test update could not have been signed, and if someone had managed to push malicious code, the same would be true, so it wouldn't have been installed onto target machines.