MongoDB Documentation, Release 3.0.4 $where Must Be Top-Level MinKey and MaxKey Queries Nested Array Queries with $elemMatch Replica Set/Sharded Cluster Validation Shard Name Checks on Metadata Refresh Replica Set Vote Configuration Validation $exists and notablescan If the MongoDB server has disabled collection scans, i.e. notablescan , then $exists queries that have no indexed solution will error. Description Equality match for either MinKey or MaxKey no longer match documents missing the field. Description $where expressions can now only be at top level and cannot be nested within another expression, such as $elemMatch . Solution Update existing queries that nest $where . Description The $elemMatch query operator no longer traverses recursively into nested arrays. For example, if a collection test contains the following document: { "_id": 1, "a" : [ [ 1, 2, 5 ] ] } In 2.6, the following $elemMatch query does not match the document: db.test.find( { a: { $elemMatch: { $gt: 1, $lt: 5 } } } ) Solution Update existing queries that rely upon the old behavior. Text Search Compatibility MongoDB does not support the use of the $text query operator in mixed sharded cluster deployments that contain both version 2.4 and version 2.6 shards. See Upgrade MongoDB to 2.6 (page 866) for upgrade instructions. Description For sharded clusters, MongoDB 2.6 disallows a shard from refreshing the metadata if the shard name has not been explicitly set. Solution Upgrade all components of the cluster to 2.6. See Upgrade MongoDB to 2.6 (page 866). Description MongoDB now deprecates giving any replica set member more than a single vote. During configuration, local.system.replset.members[n].votes should only have a value of 1 for voting members and 0 for non-voting members. MongoDB treats values other than 1 or 0 as a value of 1 and produces a warning message. Solution Update local.system.replset.members[n].votes with values other than 1 or 0 to 1 or 0 as appropriate. 865 12.2. Previous Stable Releases For mixed sharded cluster deployments that contain both version 2.4 and version 2.6 shards, this change can cause errors when migrating chunks from version 2.4 shards to version 2.6 shards if the shard name is unknown to the version 2.6 shards. MongoDB does not support migrations in mixed sharded cluster deployments.