We have a cluster with a 2.6 mongos, a 2.6 mongod as one shard, and a 2.4 as a second shard. We expect the following query to cause an error:

> db.coll.aggregate([{ "$project" : { "_id" : "$_id" }}], { explain: true })

However, the error message we get back is 'unrecognized field "cursor,"' when it really could be something more helpful like "explain is not supported for aggregation queries on mixed-version clusters."