Initial document state: {}

Oplog entries:

1: {$set: {a: '15MB string'}}
2: {$set: {a: ''}}
3: {$set: {b: '15MB string}}

Final state:

Note that at no point is the document more than 16MB so this is a valid sequence of events from the primary's perspective.

If we start applying Oplog entry 1 to the final document (which initial sync assumes is valid) we will have a document temporarily larger than 16MB which will cause an error and abort the server.