noworks.js

#!/usr/bin/env mongo --quiet

var m = new Mongo();
var db = m.getDB('test');
var t = db.getCollection('whatever');

t.find( { group:3, 'x.a':1 }, { 'x.$':0 } );
printjson(db.getLastErrorCmd());
assert.eq( 16345, db.getLastErrorCmd().code, "exclusion of positional operator" ); 

works.js

#!/usr/bin/env mongo --quiet

var m = new Mongo();
var db = m.getDB('test');
var t = db.getCollection('bunk');

t.ensureIndex({a:1},{unique: true});
t.insert({a: 1});
t.insert({a: 1});
printjson(db.getLastError());