A Soccer Ball is found in the Halo 2 Campaign level Metropolis, hidden in the rooftops. There is speculation that it was used by Bungie to test the physics engine of the game, and that they left it in the level instead of deleting it. It also appears as a Forge item available to place on certain maps in Halo 3, Halo: Reach, and Halo 4.

Halo 2 is the best selling original Xbox title released, with 8.49 million copies sold. That is over 2 Million more than the second best selling game on the system, Halo: Combat Evolved.

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.

On the level Metropolis, there is a hidden gun known as the Scarab Gun. It is extremely difficult to obtain. It is shaped like a standard Plasma Rifle, contains unlimited ammo and has the same firepower as a Scarab's main turret. In the PC version, there is an achievement for finding it.

There are 21,090 lines of dialogue in Halo 2. Most of these are randomly triggered during combat.

The PC version of Halo 2 was delayed from release due to an image of a man, who was one of the developers, mooning the camera and exposing his bare buttocks in the game's data (in an error report known as .ass).

Halo 2 received approximately 1.5 million pre orders before it's release, making it a platinum seller before it even hit the shelves. It made $125 million within 24 hours of it's release.

There were supposed to be several different types of warthogs to be driven by the player before they were cut from game development. This included an Arctic Model that would later be included in the Avalanche DLC for Halo 3 and a smaller transport ATV that would become the Mongoose in Halo 3.

There was a large, unused Flood enemy in Halo 2 called the Juggernaut. Due to conflicting design goals, it wasn't working well in the game, and so was cut. However, it was used as the basis for the Pure Form enemies in Halo 3.

"Blow Me Away" performed by the band Breaking Benjamin can be heard as the player approaches the Mausoleum of The Arbiter, as well as during the battle. Though it may sound instrumental, the vocals have simply just been made quieter.

The Mongoose was originally intended to be put into Halo 2, but was cut and later reworked for Halo 3. The Mongoose prototype did not fulfil Bungie's quality expectations for Halo 2.

In Halo 2, if you start the campaign level Gravemind on Legendary, an Easter egg can be spotted during the opening cutscene. After the Gravemind finishes his long conversation with the Arbiter and John-117, the camera fades to the High Charity station. Inside there are Brutes defending the room where the Prophets are hiding from the Elites. When the Grunt climbs the arch and the camera focuses to the crowd up ahead, you can see standing in the crowd of Grunts a hidden 2D image of a man in nothing but gym shorts looking at the floor, with his hands behind his head. Dubbed the "Half-Naked Guy", this man is Bungie co-founder Jason Jones.

In Halo 3, the Half-Naked Guy can be found again standing on a small nook in an out-of-the-way hidden area at the beginning of the last campaign level, specifically in a dark room with a lethal chasm next to the hallway for the seventh Terminal.

In the eighth campaign level Delta Halo, two Kig-Yar Minor enemies wielding Plasma Rifles can be found at the end of the tank section of the level. This is the only instance in the game where enemy Kig-Yar Minors use Plasma Rifles.