In a sharded cluster with 2 mongoses, the following interleaving will return NamespaceNotFound instead of empty result 

1. mongos 1 create the collection
2. mongos 2 runs a find or any other command that requires routing 
3. mongos 1 drops the db 
4 mongos 2 runs an aggregate

point 4. returns "NamespaceNotFound" 

I left attached a repro.

The issue has been discovered during the development of SPM-4330