@zeeg reports a user putting "Reviewers: Let me tell you a story about reviewers ..." in a summary (presumably via web UI) and then receiving an unclear error when updating with arc diff:
[2014-09-08 08:59:36] EXCEPTION: (ArcanistDifferentialCommitMessageParserException) Field "reviewerPHIDs" occurs twice in commit message! at [/home/.../.arc_install/arcanist/src/differential/ArcanistDifferentialCommitMessage.php:79]
This is because there are now two "Reviewers" fields. We should possibly try to make the parser figure this out, or at least improve the clarity of the error.