In the form at https://secure.phabricator.com/herald/test/, enter a commit like "rGITTESTaa11a8a574db", then select "Commits" on the following form and Run Tests.
The following error page results:
>>> UNRECOVERABLE FATAL ERROR <<< Call to a member function getCommitDetail() on a non-object /core/lib/phabricator/src/applications/diffusion/herald/HeraldCommitAdapter.php:191 ┻━┻ ︵ ¯\_(ツ)_/¯ ︵ ┻━┻
The bug is present in the stable branch (updated this week), but this isn't a feature I use frequently, so it could have been around much longer.