In mongos, we prevent $out to a sharded collection here. However if mongos' cache is stale and it believes the collection is unsharded or doesn't exist, then we will go ahead and establish shard cursors.

If the source collection is also sharded, then mongos will also establish a merging cursor which will run the $out on the primary shard. If this fails (in this case mongod will find the target collection to be sharded and assert here) then the original shard cursors will be leaked.