We don't do any testing specifically of rollbacks in sharding. Sharding can do special writes so we should make sure they get timestamped and rolled back correctly. A sharded rollback fuzzer that adds sharding metadata commands to the grammar would be interesting. A sharded kill_nodes suite that kills primaries and secondaries in config servers and shards would also be useful. Simply doing a sharded kill_primaries or kill_secondaries suite would go a long way.