Consider a replica set with three members at this state of oplogs:

S1: 1 2 3 4 5 6 7 8 9 (primary)
S2: 1
S3: 1 2 3 4 5

now suppose S2 starts applying the batch 2..9. It early commits

{2,4,6,8} ops. It then crashes.

after crashing, the oplog for S2 is unchanged, but writes have occurred to the datafiles for opids {2,4,6,8}

.

On restart, S2 would recover ok (if it's idempotent) if S1 is up. However, suppose S1 goes down first (perhaps permanently). Now S2 and S3 are the remaining set members on S2's restart. S3 has the latest data. After recovery we have:

S1: down
S2: 1 2 3 4 5 (+

{6,8} applied but not in oplog)
S3: 1 2 3 4 5

*However S2 has also written ops {6,8}

and they are never rolled back.