In 3.0.7 (MMAPv1), oplog entries have been observed to be in non-monotonic order on secondaries where the oplog is not a capped collection.
SERVER-20858 prevents non-capped oplogs from being created, however, this does not help if the system already has a non-capped oplog. However, in versions where SERVER-20858 isn't fixed, this is an easy situation to accidentally get into, eg. by following the oplog resize procedure but forgetting/missing the capped: true parameter, or just forgetting/missing the createCollection step entirely (the next step will create the (uncapped) collection when the last oplog entry is inserted). SERVER-20912 hints at a "preflight check". This ticket is to request that a startup warning be generated if --replSet is specified and a non-capped oplog is found. It would also be good to have rs.printReplicationInfo() print a warning if the oplog isn't capped. Both warnings should direct the user to resize their oplog.