A user on IRC is reporting receiving the error message:

ERR-CONDUIT-CORE: The owner of a revision may not be a reviewer.

...after running arc diff to create a new revision. The commit message did not appear to add them as a reviewer.

A theory I had is that Herald might be adding them as a reviewer, but the user reported that there were no transcripts. It's still possible this is the case if failing the update stops Herald from saving the transcript. This is probably worth looking into.

The message could also be more clear (e.g., enumerate all the offending parties).