The OplogReplay query flag is an internal optimization for queries on the oplog (and other capped collections) using the ts (timestamp) field. Version 2.6.0 introduced a bug whereby a query using the OplogReplay flag with additional predicates not on the ts field could return incorrect results.
Internally, replication issues queries with predicates only on the ts field, and thus is unaffected by the bug. Third-party tools or other user implementations to query or tail the oplog can break, if they use the query flag and have additional query predicates.
A query without the OplogReplay query flag would return correct results but may be less efficient as it would not employ the OplogReplay optimization.
The fix restores the behavior to ignore predicates on other fields when walking the collection backwards to find the earliest oplog entry that matches the query. This brings the behavior back in line with that of version 2.4.x. Additionally, validation was added to ensure that the predicate on ts is either $gt or $gte, as the OplogReplay optimization does not work with other operators.
Version 2.6.0 was affected by this bug.
The patch is included in the 2.6.1 production release.