See IRC. Ostensible repro steps:
- User A creates a revision.
- Herald adds Project B (which they can't see) as a reviewer.
- User A updates revision.
...producing:
> [2014-06-11 01:35:07] EXCEPTION: (ArcanistDifferentialCommitMessageParserException) Error parsing field "Reviewers": The objects you have listed include objects which do not exist (PHID-PROJ-qpp5c2zuckfsurk7nrhy).