Bug 903654 shows a case where manifests that were "invalid" (by our rules) did not break the build. It would have been easier if the annotator had thrown an exception on the missing-but-needed x-mozbuildid element. Diagnosis in bug 903654 comment 7
This annotator is part of the B2G build process.
This issue is not with add-revision.py, it is with the Releng manifest annotator. x-mozbuildid is not something that's known to repo, b2g or gonk-misc. The issue was that the releng munger was based on their being a specific string in a comment, and it failed when that comment was stripped by a change from a partner. The work that is needed is going to happen in bug 903654