Oracle Streams: ORA-0001 on dequeue? - oracle

Question

In production every now and then we get an ORA-0001 error when dequeuing a message. We use:
Solaris 10
Oracle 10g
C++ application
PROC*C with dbms_aq package for queuing operations
Queues with XML payload
We handle a large volume of messages (1K/min).
Any clue why would a dequeue result in an ORA-0001 (unique constraint) error?
Update: adding code per request.
EXEC SQL EXECUTE
DECLARE
message_properties dbms_aq.message_properties_t;
dequeue_options dbms_aq.dequeue_options_t;
message_payload xmltype;
tmpclob clob;
dynamic_sql_string varchar2(512);
BEGIN
dequeue_options.wait := :iReadTimeout;
dequeue_options.dequeue_mode := dbms_aq.REMOVE;
dequeue_options.visibility := dbms_aq.ON_COMMIT;
IF :iBuffered = 1 then
dequeue_options.delivery_mode := dbms_aq.buffered;
dequeue_options.visibility := dbms_aq.immediate;
:iNavigationMode := 0;
END IF;
IF :iDequeueOnly = 1 and :iQueueType <> 1 THEN
dequeue_options.dequeue_mode := dbms_aq.REMOVE_NODATA;
dequeue_options.wait := dbms_aq.NO_WAIT;
dequeue_options.msgid := hextoraw(:pszDequeueMsgId);
ELSE
IF :iNavigationMode = 0 THEN
dequeue_options.navigation := dbms_aq.FIRST_MESSAGE;
ELSE
dequeue_options.navigation := dbms_aq.NEXT_MESSAGE;
END IF;
END IF;
dequeue_options.deq_condition := :pszDeqCondition;
dbms_aq.dequeue(queue_name => :pszQueueName,
message_properties => message_properties,
dequeue_options => dequeue_options,
payload => message_payload,
msgid => :msgid );
IF dequeue_options.dequeue_mode <> dbms_aq.REMOVE_NODATA THEN
EXECUTE IMMEDIATE dynamic_sql_string USING OUT tmpclob, IN message_payload;
:gpoXmlClob := tmpclob;
ELSE
:gpoXmlClob := message_payload.getclobval();
END IF;
END;