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1. If the "original_replicas"  is missing for the topic/partition in /admin/reassign_partitions .  In this case, the pending reassignment cancelled will be skipped.  Because there is no way to reset to the original replicas.  The reasons this can happened  could be: 
   1. if either the user/client is tampering /admin/reassign_partitions directly, and does not have the "original_replicas" for the topic
   2. if the user/client is using incorrect versions of the admin client to submit for reassignments.   The Kafka software should be upgraded not just for all the brokers in the cluster.  but also on the host that is used to submit reassignments. 
      
      
2. If all the "original_replicas" brokers are not in ISR,  and some brokers in the "new_replicas" are not offline for the topic/partition in the pending reassignments.   In this case, it's better to skip this topic's pending reassignment  cancellation/rollback,  otherwise, it will become offline.  However,  if all the brokers in "original_replicas" are offline  AND  all the brokers in "new_replicas" are also offline for this topic/partition,  then the cluster is in such a bad state, the topic/partition is currently offline anyway,  it will cancelledcancel/rollback this topic pending reassignments back to the "original_replicas".  

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