{op: "db"} oplog entries are used to declare the presence of a database. However, applyOperation_inlock() begins to parse them as delete oplog entries, and fails because they do not have an _id field.

{op: "db"} oplog entries are used in master-slave replication. This leads to log lines like:

sync: caught user assertion 4 Failed to apply delete due to missing _id: { ns: "test.", op: "db" } while applying op: { ns: "test.", op: "db" }