Recently debugged an issue <rdar://problem/31894087> which was silently failing because of this problem. Having logs would have helped in debugging.
Created attachment 308872 [details] Proposed patch
Comment on attachment 308872 [details] Proposed patch Clearing flags on attachment: 308872 Committed r216108: <http://trac.webkit.org/changeset/216108>
All reviewed patches have been landed. Closing bug.