This is related to https://jira.mongodb.org/browse/SERVER-4157

If the _id value in the query document does not match the _id value in the update document, an empty document will be returned.

To reproduce: (in 1.8.4 or 2.0.1)

> db.test.drop()
true
> db.test.findAndModify({ "query" :

{ "_id" : 0}

, "update" :

{ "_id" : 1, "name" : "Object 1"}

, "upsert" : true, "new" : true } );
{ } // an empty document is returned
> db.test.find()

{ "_id" : 1, "name" : "Object 1" }

> db.test.findAndModify({ "query" :

{ "_id" : 2, name:"blah"}

, "update" :

{ "_id" : 2, "name" : "Object 2"}

, "upsert" : true, "new" : true } );

{ "_id" : 2, "name" : "Object 2" }

// the expected document is returned
>