After SERVER-828, it's not possible to specify a field with $ in the field as part of the projection. This is likely an edge case not handled by the Projection::validateQuery() implementation here: https://github.com/mongodb/mongo/commit/fb66c84bc7bc1ece63a65766bfea2f797f3b7121

> db.foo.drop()
> db.foo.save({a:{$id: new ObjectId()}});
> db.foo.find({}, {'a.$id': 1})
error: {
	"$err" : "Positional operator does not match the query specifier.",
	"code" : 16354
}

The reproduction steps are a bit contrived, but the practical use case is a projection of the $id field of a DBRef object.