Immediately after the mission begins, you will find a large roadway crawling with Covenant forces. Instead of fighting your way to a door that will lead you to a room with two turbine-like objects, don't go through the door and follow the road all the way to the end. Make sure to kill all covenant on the way. Once you reach the end, you will find a Marine knocking on a door wanting to get in. If you listen closely you can hear that it is actually an excerpt from Red vs. Blue where Simmons is talking to Griff trying to get into the base. Note: There is a different message for each difficulty you play on:
Two Marines copying Red vs. Blue.
Simmons and Griff from Red vs. Blue.
Church and Caboose doing a "remake" of Simmons and Griff.