emberjs/ember.js — pull request #2185 by lukemelia (2013-02-28 22:24:03+00:00)
Title: Fix issue where classNameBindings can try to update removed DOM element.

With this PR, Ember.View#registerObserver uses states to invoke observers only when there is an element for them to operate on. This avoids a class of issues that can arise when observers are triggered even though the element has been removed from the DOM. (with help from @stefanpenner and @kselden)