I was playing through the level Crows Nest in Halo 3. I decided to explore and go the wrong way. Then I saw something funny. A marine was knocking on a door. So I walked up to him and listened to the conversation. It was from Red vs Blue! I went back on all the difficulties and it changes! Here is a tutorial to do it yourselves!
Red vs. Blue reference in Crows Nest mission
When you begin the mission, you will find a large roadway with many Covenant forces. You are supposed to fight your way to a door that will lead you to a room with two turbine-like objects. Instead of going through this door, follow the road all the way to the end, killing all the Covenant on the way. Once you reach the end, there will be 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. There is a different message for each difficulty setting:
Normal: Two Marines copying Red vs Blue.
Heroic: Simmons and Griff from Red vs Blue.
Legendary: Church and Caboose doing a different version of Simmons and Griff.
I hope you find this as funny as I did!