Given a 2.6 mongos and a 2.4 mongod shard:

mongos> db.test.insert({_id : 1})
mongos> db.test.find()
{ "_id" : 1 }
mongos> db.runCommand({"update" : "test", updates : [ {q : {_id : 1}, u : {"$set" : {x : 1} }  } ] } )
{ "ok" : 1, "nModified" : 0, "n" : 1 }
mongos> db.test.find()
{ "_id" : 1, "x" : 1 }

This is not correct, as 1 document was modified.

To be consistent with SERVER-13001, mongos should omit nModified if its value can not be reliably determined due to the need to down-convert to OP_UPDATE when forwarding the update to a mongod < 2.6.