Earlier this spring, Conan O'Brien and Andy Richter goofed around in a Halo 4 voice recording booth, talking about loading crates. The duo are appearing in the game as a special Easter Egg. YouTube user GameFrontDotCom has a quick walkthrough on how to find them and hear their dialogue.
You can hear more of their dialogue in the original post from this past spring.
In case you missed it, here is Kotaku's Halo 4 review.