On ONI: Sword Base, there are two areas near where both communications relay stations are in the level that contain two invisible switches. They both need to be activated at the same time, meaning only two players can activate them. After both players activate the switches by holding X, a new objective appears called "Reach Racer" and spawns both players near the main base's exit, along with two Warthogs. From that point on, it's a race around the map and back to the starting point while crossing several checkpoint flags along the way. Once finished, the map resets, but you can replay this minigame as many times as you like by re-activating the two switches. This mode is a reference to the Ridge Racer series.

There's an Easter egg that can allow the player to pilot a Pelican and a Phantom. This can be done in the New Alexandria level, after the player activates a switch and then passes through a ringed archway a couple of times with a Falcon and a Banshee respectively. The vehicles will then automatically disappear once the player exits them.

After Halo 3 was finished, Bungie considered developing Halo 4, but they decided to develop Halo: Reach instead.

In Halo 2, Halo 3, and Halo: Reach, the bomb in the Assault multiplayer game mode sometimes has a hidden message written on it. The possible messages include "Thus I Refute Thee", "Kiss It Goodbye", "Hold Onto Yer Butts", and a hieroglyph of a Spartan helmet along with the two-fingered peace sign and "xoxo & hearts".

The first message, "Thus I Refute Thee", is a reference to the logical fallacy of appealing to the stone originating from the English writer Samuel Johnson, who during an argument about how it was impossible to refute idealism, kicked a large stone and said the quote. This quote was put on the bombs in Halo 2 by Bungie senior employees Robert and Lorraine McLee, in response to a conversation about Marathon and philosophy between two members of the Halo fan website Team 7HR33, who agreed that it would be "the uber-quip" to inscribe on the side of a gun.

The achievement in Halo Reach and Halo: Combat Evolved Anniversary called "Hell's Jerome" is a reference to Bungie's head of security, Jerome Simpson, after a photo of him (suggesting that he had damaged a car with a gravity hammer) went viral online.