It is possible for getMajorityReplicationLag to return negative values because it doesn't currently retrieve the values for last applied and last committed atomically.

The _shardsvrReshardingOperationTime command will error out when this happens because it will fail [this validation|
https://github.com/10gen/mongo/blob/fc8633645a4a790ccf7ac30d54b95408267731f5/src/mongo/s/request_types/resharding_operation_time.idl#L77]