If Phabricator is used to import a Mercurial repository which has the blackbox extension enabled, the import will never complete, and the revision log at https://example.com/diffusion/HG/ will show an extra revision at the top, "rHGwarning: cannot write to blackbox.log: Permission denied"; clicking the link results in a HTTP 404 error.
The blackbox extention, documented at https://mercurial.selenic.com/wiki/BlackboxExtension creates a log of operations into .hg/blackbox.log .