In a mixed version sharded cluster, multiple shards can participate in the merge, each using different sorting semantics. Consider an aggregation that targets two shards, one is version 3.6 and one is version 3.4, then is merged by the 3.6 node. The results from the 3.4 node will come back in a different order than they would on the 3.6 node, but the 3.6 merging node will assume they are both sorted the same way, incorrectly using a 'merge presorted streams' algorithm, for streams that aren't actually sorted.