At the beginning of initial sync, we take note of the current most recent document in the primary's oplog. After cloning the data, we apply the oplog for all entries newer than and including the entry that was noted to start with. We should only apply entries newer than the initial one. This can cause problems when users do complex backup/restore procedures where the document referred to by the most recent oplog entry ceased to exist because of modifications to the file system outside of MongoDB.