We have a bunch of git repos all hosted externally. I haven't added them all to Phabricator.
I made a change in a repo that was not yet added to Phabricator - the change was to add the .arcconfig file. So Arcanist worked, but I did NOT add the repo to Phabricator with Diffusion. Then I typed arc diff ... and created a Diffusion code review in Phabricator for a repo that didn't actually exist there.
I worked fine other than there was absolutely no mention of what repo the change was for in Differential. The review noticed this and pointed out the problem to me. Arcanist/Differential should at least warn about a missing repo - we'd probably like it to completely deny the code review.