Seems like the test description and the comments in the test are off a bit:

https://github.com/kselden/ember.js/blob/d4fd799020c995e784944139c9fd61895d5b5c4f/packages/ember-runtime/tests/mixins/array_test.js#L451

> // Observer fires second time when new object is added

Should be: "Observer fires first time when new object is added"

https://github.com/kselden/ember.js/blob/d4fd799020c995e784944139c9fd61895d5b5c4f/packages/ember-runtime/tests/mixins/array_test.js#L453

> // Observer fires third time when property on an object is changed

Should be: "Observer fires second time when property on an object is changed"

https://github.com/kselden/ember.js/blob/d4fd799020c995e784944139c9fd61895d5b5c4f/packages/ember-runtime/tests/mixins/array_test.js#L456

> "observers should only be called once"

Not sure on this one.  Are we saying that they should only be called once per change? If so the current wording is OK. If not then it should be twice or a better message.