To test the $currentOp output for sharded transactions, sharding/aggregation_currentop.js starts a transaction through mongos using "snapshot" read concern, which can occasionally with SnapshotTooOld, which mongos can't retry on until SERVER-39704 is completed. The test doesn't rely on the particular read concern level of this transaction, so to avoid this problem the transaction can use a read concern level that does not require a global snapshot - majority, local, or none.