I upgraded a 6 month old installation to HEAD. Can it be related? Didn't experience any other issues.

@chad, thanks for the comments. There is no doubt that Differential mode is a proffered way to do a code review.
However it has some issues:

1. Using manually uploaded diffs is nice, but not an option for a daily routine.
2. While Arcanist is not hard to use, it requires though some magic to make it work with PyCharm, Eclispe or any other IDE being used. It should be convenient for people to use and hidden as much as possible. If you could just replace "arc" with "hg" that would be a perfect match.

@eadler - yes T731 looks similar.
@chad, we use only Audits at this point. The reason we don't use pre-commit workflow is mainly due the fact that we didn't want to introduce a new Source Code Control system, besides what we already use - mercurial. Our team works on many platforms and each developer uses its own IDE, so at this point we didn't want to convert everyone to Arcanist.
In our current flow everyone that want his commits to be reviewed, uploads them to a "review" branch. After a successful review, the branch is merged with the master branch.
Additional thing I find lacking is a way to compare ("diff") a series of commits. Think of a following flow:

1. Alice pushes commit A and sets Bob as Auditor.
2. Bob comments on commit A and rejects the commit.
3. Alice pushes commit B to address Bob's comments.