If there is heavy write locking on the master, the slave may receive data from the oplog before it is committed on the master.

1) Some data which is lost by the master on restart (because it was not committed) may be present on slave. I have observed this in testing.
2) The slave may record a syncedTo point that is uncommitted on the master. On restart the master will not return the expected op time to the slave and the slave will generate a sync exception forcing the need to reclone. I haven't seen this in testing, but it may be possible.