Description:
'Commit failed with exception "Session not transacted". Will rollback current transaction.', probably due to the incorrect use of JMS Topic Subscriber.
Symptoms:
The following lines are seen in the BW engine logs:

2012 Apr 04 10:33:25:509 GMT +8 BW.jmst Debug [BW-Core]  Job-3021 Error in [Process Definition (2).process/Group/end] Commit failed with exception "Session not transacted". Will rollback current transaction.
2012 Apr 04 10:33:25:510 GMT +8 BW.jmst Debug [BW-Core]  <= Process Definition (2).process/Group/end.eval(Job-3021) returned ERROR
2012 Apr 04 10:33:25:514 GMT +8 BW.jmst Error [] null Job-3021 Error in [Process Definition (2).process/Group/end]
Commit failed with exception "Session not transacted". Will rollback current transaction.
    at com.tibco.pe.plugin.transaction.JMSTransactionResource.commit(Unknown Source)
    at com.tibco.pe.core.TransactionGroup.eval(Unknown Source)
    at com.tibco.pe.plugin.Activity.eval(Unknown Source)
    at com.tibco.pe.core.TaskImpl.eval(Unknown Source)
    at com.tibco.pe.core.Job.a(Unknown Source)
    at com.tibco.pe.core.Job.k(Unknown Source)
    at com.tibco.pe.core.JobDispatcher$JobCourier.a(Unknown Source)
    at com.tibco.pe.core.JobDispatcher$JobCourier.run(Unknown Source)
Cause:
Incorrect use of JMS Topic Subscriber. Acknowledge Mode should be set to Local Transactional (for JMS Transaction group) or Transactional (for XA Transaction group), if the JMS Transaction group or XA Transaction group is created and "Include In Transaction" of the group is set to JMS Topic Subscriber.

'Commit failed with the exception, "Session not transacted". Will rollback current transaction.'

Set Acknowledge Mode to Local Transactional or Transactional. There is no need to use Confirm activity in this case. The message is acknowledged when the transaction commits.