When you create and run an On-demand Archive job, occasional failures may occur.  Some failures are not terminal, so the job actually completes, but some errors can be recovered. In those situations, you will be presented with the option to True-up that job.  Access to the true-up capability will be available from two places in the list of On-demand Migration & Archiving Jobs (Settings -> Content Lifecycle): 

 

On the main list of jobs:

 

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Within the details of the job itself (which you can access by clicking on any individual job in the list)

 

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Until the failed job is either canceled (using the "Don't archive" option) or has completed the true-up, any subsequent jobs created with the exact same source and destination will not start.