Angst and the Angular Module System
Google's AngularJS is one of the most popular web frameworks out there, but it comes with its share of criticism (performance around two way binding being one of the more prominent complaints). Google responded by releasing Angular 2 (and, as of now, 4, 5 and 6), which addressed some of the issues but at the cost of being drastically incompatible with its predecessor, with no realistic way to upgrade except through pure elbow grease. Angular 2+ might as well be a completely different framework.