The mongodump tool fails to dump the local.oplog.rs and local.oplog.$main collections when the --dbpath argument is specified and quits with an error.
Users cannot dump an oplog directly from the files but need to start a mongod instead.
Affected users can create the dump from a running mongod instance and avoid the --dbpath option as a workaround. Another workaround is to pass in a query with a ts field as the --query argument.
Version 2.6.1 was affected by this bug.
The fix is included in the 2.6.2 production release.
The oplogReplay query option is only set if the query contains a ts field.