mongoS will crash with an invariant failure when:
- The initial resume token expected by mongoS is the split event fragment.
- On the shards, DSCSCheckResumability lets the base event through expecting it to be split.
- But there is no split stage, and the base event is returned to mongoS directly as-is.
- But the event's resume token is BEFORE the split, so mongoS has gone back-in-time.