Currently CollectionRoutingInfoTargeter remembers a response error in _lastError and assumes that there could be only one type of error, which is controlled via dassert. It's possible that different shards can send different types of errors (StaleConfigInfo, 
StaleDbRoutingVersion).  If the error types are processed in order, like in src/mongo/s/write_ops/bulk_write_exec.cpp, it can come to the 
CollectionRoutingInfoTargeter::noteStaleDbVersionResponse after processing of noteStaleCollVersionResponse and dassert throws. StaleShardVersion and StaleDbVersion errors are handled in the same way and can be unified to one StaleRouting type.