assertion failure 9.3.4

With a client's code I have just managed to produce the following 
assertion failure on 9.3.4:
   2014-04-15 01:02:46 GMT [19854] 76299: LOG:  execute <unnamed>:   select * from "asp_ins_event_task_log"( job_id:=1,
event_id:=3164,  task_name:='EventUtcComputeTask', task_status_code:='VALID'   , task_start_utc:='04/15/2014
01:02:44.563',  task_end_utc:='04/15/2014 01:02:44.563')   TRAP: FailedAssertion("!(update_xact == ((TransactionId)
0))",File:   "/home/andrew/pgl/pg_9_3/src/backend/access/heap/heapam.c", Line: 5873)   2014-04-15 01:02:46 GMT [11959]
11:LOG:  server process (PID 19854)   was terminated by signal 6: Aborted   2014-04-15 01:02:46 GMT [11959] 12: DETAIL:
Failed process was   running: select * from "asp_ins_event_task_log"( job_id:=1,   event_id:=3164,
task_name:='EventUtcComputeTask',task_status_code   :='VALID', task_start_utc:='04/15/2014 01:02:44.563',
task_end_utc:='04/15/201401:02:44.563')   2014-04-15 01:02:46 GMT [11959] 13: LOG:  terminating any other   active
serverprocesses
 


When running without assertions, the client reports experiencing tables 
with duplicate primary keys among other things. It's apparently quite 
reproducible.

I'm digging into this, but it's a nasty bug and any useful thoughts 
would be appreciated.

cheers

andrew