When a mongos with stale db version tries to send a write to an unsharded collection to the wrong shard, it will trigger the onImplicit create error since the collection does not exist on the shard. It will perform a refresh on the collection version and retry again, but since the database version was not refreshed, it will be sent to the same wrong shard and hit the same error again. This will keep on happening until it ran out of retries.