Essentially, the issue is what happens when a task is assigned or attached to a user unknown to the human-task instance.

Simple process definition: start->human task->end with potential owner, that is not known to the HT service is run in the JBPM Console.
When the process instance starts, exception occurs in the server.log (see an attachment).

There is a root cause:

java.sql.SQLException: Integrity constraint violation - no parent FK27A9A59E619A0 table: TASK in statement [update Task set allowedToDelegate=?, taskInitiator_id=?, priority=?, activationTime=?, actualOwner_id=?, createdBy_id=?, createdOn=?, documentAccessType=?, documentContentId=?, documentType=?, expirationTime=?, faultAccessTmy pointype=?, faultContentId=?, faultName=?, faultType=?, outputAccessType=?, outputContentId=?, outputType=?, parentId=?, previousStatus=?, processId=?, processInstanceId=?, processSessionId=?, skipable=?, status=?, workItemId=?, OPTLOCK=? where id=? and OPTLOCK=?]

From the users point of view:
1. The task cannot be seen by unknownUser - that's right, it should not be.
2. There is the process instance waiting for the task completion. The only way is to terminate this process instance -> the user can lost his data.
3. There should be an option to assign the task to other user which is known to the HT service (for example by some Task Server admin). But this is not possible - the task is not stored in database and therefore, it can't be seen by any user.
4. If there is more than one potential owner specified in the process definition and only one is unknown to the HT service, still even the known owners are unable to access the task.

Steps to Reproduce:
1. run the attached BPMN2 process (assuming the "unknownUser" is not defined in the running task service) in JBPM Console
2. see server log for the exception