Last November, we moved one of our github repositories from being public to private. As a result, diffusion broke on that repo (because the repo wasn't set up to need credentials to pull).
This broken-ness was duly reported at https://phabricator.khanacademy.org/diffusion/AC/manage/status/, which had a red X for "importing," but unfortunately we don't make a habit of checking that page. As a result, the repo was "silently" broken. It took us until now to finally get fed up with revisions not auto-closing, to track down what was going on.
Maybe most installs would notice faster if diffusion hadn't been updating a repo for a few months, but having a more prominent notification (perhaps in the status dropdown) when there's a diffusion error such as this one, would have helped us for sure.