I found a problem with Linked MediaWiki accounts (T574). I figured out it belonged in this project so I visited https://phabricator.wikimedia.org/tag/phabricator/ and clicked [+ New Task]. After writing a Chekhov short story I clicked [Create Task], only to fail with
"Access Denied: Application Maniphest You do not have permission to edit task status."
Expected: fail earlier, or don't show the [+ New Task] button to users who don't have the permission.
Workaround: join the Triagers group, even though the promotion on the phabricator.w.o homepage says "Do you miss managing Priority, Status, and Assigned fields in tasks?" rather than "Do you want to be able to submit bugs?". Then reload the failure page to retry your form submission.