Hosting your JS through a CDN is good. Whether it's a public or a private CDN, CDNs occasionally are unreachable (downtime, firewall/proxy restrictions). It's possible to defensively 'fall back' on a secondary source, possibly locally hosted. e.g.:

<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.4.0/angular.min.js"></script>
<script>window.angular || document.write('<script src="\/fallback_angular_1.4.js"><\/script>');</script>

While I've seen this on smaller web properties, and there are many blog posts extolling the practice, I've not seen it on anything particularly high-traffic. If they exist, which are they?