2.4.1 here. I spoke to @milkie about this in dev chat.

> Jeremy: should the massert (no ts field in query) in [FindingStartCursor::init()](https://github.com/mongodb/mongo/blob/master/src/mongo/db/repl/finding_start_cursor.cpp#L162) happen for any [OPQUERY](http://docs.mongodb.org/meta-driver/latest/legacy/mongodb-wire-protocol/#MongoWireProtocol-OPQUERY) with the OplogReply flag missing the ts criteria?
> Eric: i don't think you can hit that code if you don't set OplogReplay?
> Eric: OplogReplay just means use a FindingStartCursor for the query, and also do the work for updating your location for write concern, based on where the cursor is for the query.
> Eric: oh I misread your question.  yes, if you make an OplogReplay query with no ts field, it will massert.

Note: In 2.4.1, FindinstStartCursor was located in [mongo/db/oplog.cpp](https://github.com/mongodb/mongo/blob/1560959e9ce11a693be8b4d0d160d633eee75110/src/mongo/db/oplog.cpp#L547) instead of [mongo/db/repl/finding_start_cursor.cpp](https://github.com/mongodb/mongo/blob/master/src/mongo/db/repl/finding_start_cursor.cpp#L162), but the `massert()` is the same.

I would reckon on some environments, we're simply not setting the flag bit correctly. Thoughts?