FWIW this problem was reported also by Andrew Sackville-West at http://www.postgresql.org/message-id/20140731233051.GN17765@andrew-ThinkPad-X230 I strongly suspect now that the problem is related to the locking of updated versions as heap_lock_tuple_updated, and perhaps the internal locking done by EvalPlanQual. Haven't traced through it.
Is there anything I could tell the developer to do (he's on Mac) so he could provide more information?