The following scenario can occur:
- Migration of a chunk from shard X to shard Y completes with commit/abort and the migration coordinator on shard X persists the decision
- Shard X is removed and shut down before the migration coordinator updates config.rangeDeletions on shard Y
- Shard Y is left with document in config.rangeDeletions corresponding to that migration with a 'pending: true' flag
The presence of this document will permanently prevent any migrations to shard Y for chunks overlapping this chunk.
In the case where the migration aborted after migrating some documents to shard Y, this will also leave documents in this chunk orphaned on shard Y.