Currently mongos validates that the shard key as it sends data to the shard but the shard doesn't ensure that the shard key value is correct for itself. It is possible that a bug in routing could cause a document to be sent to a shard that is incorrect and the shard will not have an error (allowing the insert).

This enforcement should only apply to sharded connections so that maintenance and admin work can be done directly to the shard.

If this is done this should also be applied to inserts on updates (upsert:true case) as well.