angular.js/docs/content/error/$q/qcycle.ngdoc

@ngdoc error
@name $q:qcycle
@fullName Cannot resolve a promise with itself
@description

Occurs when resolving a promise with itself as the value, including returning the promise in a
function passed to `then`. The A+ 1.1 spec mandates that this behavior throw a TypeError.
https://github.com/promises-aplus/promises-spec#the-promise-resolution-procedure

```
var promise = $q.defer().promise;

//bad
promise.then(function (val) {
  //Cannot return self
  return promise;
});

//good
promise.then(function (val) {
  return 'some other value';
});
```