The config.rangeDeletions collection stores a document of the following form to track range deletion tasks needing to be performed:

{
    "_id" : UUID("78447b8a-84d6-4555-a97d-b4bc2d708e29"),
    "nss" : "test4_fsmdb0.fsmcoll0_29",
    "collectionUuid" : UUID("1eafdb97-2114-4b72-a6ce-34e96aa3df1a"),
    "donorShardId" : "shard-rs1",
    "range" : {
        "min" : {
            "a" : 400
        },
        "max" : {
            "a" : { "$maxKey" : 1 }
        }
    },
    "whenToClean" : "delayed"
}

The "range" field of this document is left unmodified after a user has successfully run the refineCollectionShardKey command. This means if the following sequence of events occurs, then a "RangeOverlapConflict: Requested deletion range overlaps a live shard chunk" error will prevent the range deleter from ever deleting the range of orphan documents.

1. The test4_fsmdb0.fsmcoll0_29 collection is sharded on {a: 1} and has chunks
   • {a: MinKey} -> {a: 100}
   • {a: 100} -> {a: 200}
   • {a: 200} -> {a: 300}
   • {a: 300} -> {a: 400}
   • {a: 400} -> {a: MaxKey}
2. A migration begins for chunk {a: 400} -> {a: MaxKey} from shard-rs1 to shard-rs0.
3. Migration completes successfully but the primary of shard-rs1 steps down before the range deletion task completes.
4. User runs the refineCollectionShardKey and changes the shard key of the test4_fsmdb0.fsmcoll0_29 collection to {a: 1, b: 1}. The chunks are therefore augmented to be
   • {a: MinKey, b: MinKey} -> {a: 100, b: MinKey}
   • {a: 100, b: MinKey} -> {a: 200, b: MinKey}
   • {a: 200, b: MinKey} -> {a: 300, b: MinKey}
   • {a: 300, b: MinKey} -> {a: 400, b: MinKey}
   • {a: 400, b: MinKey} -> {a: MaxKey, b: MaxKey}
5. The newly elected primary of shard-rs1 attempts to schedule the range deletion task still present in its config.rangeDeletions collection. Since the {a: 300, b: MinKey} -> {a: 400, b: MinKey} range is considered to partially overlap with the range {a: 400} -> {a: MaxKey} because {a: 400} < {a: 400, b: MinKey}, if shard-rs1 happens to own that range then it'll refuse to perform the range deletion for {a: 400, b: MinKey} -> {a: MaxKey, b: MaxKey}.