UK chat show host Jonathan Ross voices one of the marines that helps Master Chief in the campaign, although it is an extremely rare occurrence for one of his lines to come up.
In an out of the way hidden area at the beginning of the last campaign level, there is a hidden 2D image of a man in nothing but gym shorts, dubbed the "Half-Naked Guy". This man is Bungie co-founder Jason Jones. This same image can be seen during a cutscene in Halo 2.
Press down on both thumbsticks during the loading screen on December 25 will display a wider view of the Halo ring. Looking closely, it's possible to see the words "Happy Birthday, Lauren!" spelled out on the ring's surface.
In the game's last level, Spoiler:shortly before reaching the Frigate, there is a Grunt hidden at the end of a small pathway around some rocks. The Grunt will taunt Master Chief, and then proceed to plead for his life. However, if Thel Vadam sees the Grunt, he will kill him before the Grunt has a chance to finish talking.
The Beta release included an achievement called "Cheneymania", awarded for killing 10 opponents in a row with the shotgun without dying. This is a reference to former United States vice-president Dick Cheney, who accidentally shot someone with a shotgun.
Although Spartans and Elites have different body shapes, they use the same hitbox. Because of this, there's a glitch where bullets, grenades, and Needler rounds will pass unimpeded through part of an Elite player's neck without dealing damage to the player.
In the level "The Covenant" on Normal or higher difficulty, the Windows voice program "Microsoft Sam" can be found commenting on the game and it's developers on the right ledge of the Citadel, whilst on the left ledge can be heard the song "Siege of Madrigal" from Bungie's previous game 'Myth'.
Like in Halo 2, the bomb in the Assault multiplayer game mode will sometimes have a hidden message written on it.
In the multiplayer map "Sand Trap," it is possible to flip an Elephant over. When A Spartan/Elite tries to flip it back over, it will say "Hold RB to flip... wait, what? How did you do that?"
On the level "Crow's Nest", you can find a solder knocking on a door wanting ammo, while the man behind the door will ask for a password to get in. Depending on what difficulty you are playing, the conversation is different. These conversations are performed by the online comedy group, Rooster Teeth, famous for their Halo series, Red vs. Blue.
There is a family of three monkeys and a teddy bear hidden in the first playable level of the game.