In a clustered deployment with DocumentNodeStore on MongoDB it may happen that concurrent updates on the new ordered index fail because of conflicts.
A common use case is maintaining an ordered index on a last modified date. When nodes with such a date are added concurrently on multiple cluster nodes, then all of them will try to update the ordered index at one end of the key list. The DocumentNodeStore will perform a couple of retries but there is no guarantee that the cluster nodes will sync within that time frame or some other session conflicts yet another time.
A possible workaround is to declare the index as asynchronous.