Regardless of whether or not a source capped collection has an _id index, our replication code will create an _id index on the target collection to make op replay detection efficient. As a consequence, insertion into the target collection could potentially be slower than insertion into the source collection, eventually causing the slave to become out of sync if there is a high frequency of writes to the master. One way to resolve this would be to detect replays by verifying the _id of the newest element in the capped collection (the one which will precede the new object to be added if the new object described in the op is in fact added).