Implicitly replicated collections are a set of internal collections that participate in oplog replication partially by only replicating deletes. Inserts to these collections are implicit by other writes to potentially different collections.

This behaviour however, can result in inconsistency during secondary oplog application. Suppose the following scenario:

• T1 writes to collA, which implicitly inserts to impA (implicitly replicated)
• T2 deletes the insert performed to impA

In this case both T2 and T1 will be executed in parallel during oplog replication on the secondary if they are part of the same batch. This might lead to the delete operation being performed before the insert. As a result, the document will not exist on the primary but survive on the secondary.