I expect the two findOne queries here to return the same result. This seems like a bug:

$ mongo
MongoDB shell version: 2.0.2
connecting to: test
> db.test.drop()
true
> db.test.insert(

{ x: 1, y: 2 }

)
> db.test.findOne(

{ x: 1, y: 2 }

)

{ "_id" : ObjectId("4f0885cd61d25fd4a5813ae2"), "x" : 1, "y" : 2 }

> db.test.findOne({ $and: [

{ x: 1 }

,

{ y: 2 }

] })
null