Summary

When taking an incremental backup the diff had an offset greater than the filesize for the oplog.rs. This issue was seen with mongod version 5.0.6 on macOS. This reveals a potential bug in WiredTiger.

The database files and backup logs can be found in the linked HELP ticket. 

Timeline of events

1. Started a 2 node replica set using mongod version 5.0.6.
2. Created multiple collections using the .mongorc.js script attached here.
   1. 2 collections - each around 1.5 GB in size
   2. 1000 smaller collections - each around 1 MB in size
3. Took a snapshot of this data.
4. Updated nearly all of the documents in the 2 larger collections using the .mongorc.js script.
5. Took an incremental snapshot of this data. The snapshot seemed to work fine, but noticed that many diffs were reported for the oplog.rs collection with offsets past file size. (The diffs for the 2 large collections did NOT go past fileSize.)