If a recipient shard has an entry for the collection, it does not ask the donor for the UUID.

So, it can accept a chunk from a different incarnation of the collection, resulting in a UUID inconsistency between the sharding catalog and the collection on the recipient shard.

This garbage state from a previous incarnation of a collection can exist on a recipient for a variety of reasons, such as:

• a movePrimary failed after updating the sharding catalog but before dropping the collection from the old primary
• orphaned chunks from the previous collection still exist
• a user manually created the collection directly on the shard