When querying documents using $within, in some situations some results are omitted:

Here is a test case:
> db.pos.remove()
> db.pos.save(

{ loc: [ 5000000, 900000 ], id: 1}

)
> db.pos.save(

{ loc: [ 5000000, 900000 ], id: 2}

)
> db.pos.ensureIndex(

{ loc : "2d" }

,

{ min : -21000000 , max : 21000000 }

)
> db.pos.save(

{ loc: [ 5000000, 900000 ], id: 3}

)
> db.pos.save(

{ loc: [ 5000000, 900000 ], id: 4}

)
> db.pos.find({"loc" : {"$within" :

{"$center" : [[5000000, 900000], 1.0]}

}}).count()
4
> db.pos.find({"loc" : {"$within" :

{"$center" : [[5000001, 900000], 5.0]}

}}).count()
1
> db.pos.find({"loc" : {"$within" :

{"$center" : [[5000001, 900000], 5.0]}

}}).toArray();
[

{ "_id" : ObjectId("4c173306f5d9d34a46cb7b11"), "loc" : [ 5000000, 900000 ], "id" : 4 }

]