I thought about it a bit more and I think the current behavior is correct, because:

1. the `fn` callback should not be in the flow of the promise - if there's an exception in the callback it should still notify

2. if the creator really wants to transform the counter, it is still possible:

```js
var p = $interval(function() {...});
return p.then(null, null, function(counter) {
  return 'transformed counter';
});
```