I can confirm that videos, articles, images, 404 pages are all served by a CDN. Our 404 pages were not cached by the CDN. Allowing them to be cached reduced the origin penetration rate substantially enough to amount to a 66% reduction in outgoing bandwidth over uncached 404s.

Edit: This is not to say that our 404s were not cached at our origin. Our precomputed 404 was cached and served out without a database hit on every connection, however this still invokes the regular expression engine for url pattern matching and taxes the machine's network IO resources.

As are the 404 pages, according to the comment.