Consider a replica set chaining as follows: P -> S1 -> S2 Consider 4 points in time: t1 -> t2 -> t3 -> t4 Current time is t4 . Primary P  has oplog entries back to time t3 . Secondary S1  has oplog entries further back to t1 due to replication lag. Secondary S2 requires oplog entries back to t2 . Secondary S2  realizes its sync source S1  is lagged by more than 30 seconds and re-evaluates sync sources. S2 disqualifies S1 from consideration because it is more than 30 seconds behind. S2 considers P but disqualifies it because it only has oplog entries back to t3 . S2 cannot find a valid sync source and falls off the oplog even though S1 has the oplog entries it requires to catch up.