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:='04/15/2014 01:02:44.563')
2014-04-15 01:02:46 GMT [11959] 13: LOG: terminating any other
active server processes

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