I have seen this happen from time to time. Usually happens to me more when I am modifying a workflow of a main process rather than something like email monitoring but it still happens. I don't know for sure but I always guessed that it had something to do with a change that was made that was not part of an original process which then caused the process to get confused with the particular item trying to pass through a workflow. I've also seen it happen if the server gets restarted at the exact point an item was being ingested. (Happens a little less so far in 7.5 as the processes are a bit faster).
You can close the ticket which will remove it from view and the processes through the DB with the following:
-- The following SQL script changes orphan tickets to Closed.
-- Before using, it is recommended to back up the ProcessManager database before proceeding.
--Note: Change the 'ReportProcessID' value to the processid (ticket number) you wish to close
--For example, 'IM-000400'
--You mamy also change the text for close_code in the second block ro reflect why it was closed of you so desire.
USE ProcessManager
UPDATE [ProcessManager].[dbo].[ReportProcess]
SET [Result] = 'Closed'
WHERE ReportProcessID = 'IM-000400'
UPDATE [ProcessManager].[dbo].[ServiceDeskIncidentManagement]
SET [close_code] = 'Closed Orphan'
WHERE Process_ID = 'IM-000400'
UPDATE [ProcessManager].[dbo].[Task]
SET [IsCompleted] = '1'
WHERE WFTaskNumberPrefix = 'IM-000400'