$set: {} is now rejected by the update framework, but worked as a noop prior to 2.6

> db.runCommand({ findAndModify: "foo", query:{_id: ObjectId("536beb62fe2ac487fb241614")}, update:{$set : {} }, upsert:true} ) { "value" : null, "errmsg" : "exception: '$set' is empty. You must specify a field like so: {$mod: {<field>: ...}}", "code" : 9, "ok" : 0 } > db.foo.update({_id: ObjectId("536beb62fe2ac487fb241614")},{$set:{}},true) WriteResult({ "nMatched" : 0, "nUpserted" : 0, "nModified" : 0, "writeError" : { "code" : 9, "errmsg" : "'$set' is empty. You must specify a field like so: {$mod: {<field>: ...}}" } })