By default, the intermediate cache is non-transactional, so a topology change will cause write commands to be retried. Because a PutKeyValueCommand(K, DeltaAwareList) command is not idempotent, a retried command will append extra intermediate values to the list.

The M/R framework tries to guard against this by waiting for all the nodes to initialize the intermediate cache before starting the reduce phase, but it cannot guard against nodes joining or leaving during the reduce phase.