In early pre-release material for the game, such as an advertisement at the end of the novelization "Halo: The Fall of Reach", Halo was anticipated to be released with a Teen rating by the Entertainment Software Ratings Board. However, the ESRB re-evaluated the game later during development and gave it a Mature rating, telling Bungie they would need to make certain changes to the game to revert it back to a Teen rating. According to former Bungie community manager Matthew Soell:

On the first part of Halo (the second campaign level), if the player manages to get on top of the cliffs, a unique scripted trigger volume called "gotohell_beatch" will activate, which disables all player inputs, including opening the menu and leaving the game. This is the only area in the entire game that uses this method to prevent the player from escaping the level. According to game designer Jaime Griesemer, this was put in because a couple weeks before the game's release, some playtesters figured out how to climb the sides of the cliff, thus skipping parts of the mission. He did not have enough time to implement a physical block before the hands-off deadline for development, which resulted in him writing this script.

In the campaign levels "Assault on the Control Room" and "343 Guilty Spark", a glitch can be performed where if you hold down the button to throw a grenade right as a cutscene showing you arriving on the Pelican starts, you will stay inside the Pelican despite map scripting normally forcing you to jump out. The ship will then fly off out-of-bounds. If this glitch is done in the former mission, you can exit at any time and explore, but if you do not jump out in time, the Pelican will fly through a kill barrier and kill you. If done in the latter mission, you can safely get out at any time and explore with no risk of death. If you travel northeast from where the Pelican landed, you may eventually encounter a lone Marine, Private Mendoza from later in the mission, standing around in the empty area doing nothing. In Anniversary Edition, this Marine is slightly hidden behind a tree (several were added to this area in this edition), but he can still be seen as the trees do not have collision-detection, allowing you to walk through them. While it may seem that him standing around doing nothing is just an Easter egg, his presence here is actually a pre-determined location for his model to spawn in at before he is needed later in the level so there are no memory allocation spikes that would occur if his model was just loaded in and destroyed from the level. This is part of a common game programming pattern called "object pooling" where multiple objects are pre-initialized to be in use, and once their usage is finished are set to no longer be in use. The position of Mendoza's model is changed to be used in a pair of cutscenes during the mission.

It's notable that Mendoza's model has very minimal AI unlike other NPCs in the stage. Normally, if you use mods to possess any other character in the level and play as them, the game will recognize that the functions assigned to that character are being overridden by the player character and the level will crash. If you use mods to possess Mendoza's model however, the game will not recognize his functions have changed and you can play through the entire mission as him. He has less functionality than Master Chief, he freezes when he is hit due to him playing an animation central to his original AI, and will die in a few hits, but with the God Mode invincibility cheat on he can complete the level. Although designer Jaime Griesemer mentioned that there was a playable Marine unit named "future soldier" in an earlier RTS prototype of Halo titled "Monkey Nuts"/"Blam!", it's unknown if Mendoza specifically was ever planned to be playable.

In 2022, an Easter egg was discovered on the title screen of the Beta 1749 build of the game dating back to August 15, 2001. A hidden legible message can appear among the 8-bit text reading:


This presumably refers to staring directly in front of the screen to read the tiny message.

For the premiere demo of Halo: Combat Evolved at MacWorld 1999, Marty O'Donnell and Michael Salvatori were tasked to create music for the demo within three days. They were directed to make the song that would become the Halo series' main theme sound "ancient, mysterious and epic". While driving to Salvatori's home, O'Donnell brainstormed the song and its core melody:


The song was recorded the day prior to MacWorld 1999 by three jingle singers, O'Donnell and Salvatori, accompanied by a string sextet of four violins and two cellos. O'Donnell requested one of the jingle singers to perform the Qawwali-like voice solo during the string melody, but upon hearing O'Donnell's example, it was decided O'Donnell would sing the solo instead.

In the level "Assault on the Control Room", by flying a Banshee onto the 2nd platform above the control room and then standing at the edge on the right, the song "Siege of Madrigal" from an earlier Bungie game, Myth: The Fallen Lords, can be heard.

The Elite's phrase "Wort! Wort! Wort!" is actually Johnson saying "Go! Go! Go!" in reverse.

When Halo was changed to a first-person shooter, Microsoft suggested adding "Combat Evolved" to the title to make it more descriptive, like the military games they were competing with.

After beating the game on the Legendary difficulty, a special cutscene plays. Spoiler:Sergeant Johnson is seen grappling for an assault rifle with a Covenant Elite. When the Pillar of Autumn's self-destruct sequence is activated, Johnson says to the Elite "This is it baby. Hold me." The two then hug while the Halo ring explodes. An updated version of the cutscene is also present in Halo: Combat Evolved Anniversary.

Halo was originally planned as a Mac exclusive, and was also an RTS similar to Myth, a previous game by Bungie, and later unveiled at Macworld as a third-person shooter. This was until Microsoft bought Bungie in 2000. The game was also considered for release prior on the PlayStation 2 until the partnership with Take 2 ended.

There is a mistake on the 12th page of the game's manual. The blueprints for the Plasma Rifle have the blueprints for the Needler placed behind them.

Halo: Combat Evolved was originally going to be a Real Time Strategy game, and then a Third person shooter before eventually becoming a First Person Shooter.

During Halo's first European press tour, the demo computer blew up into smoke. Joe Staten, lead writer and cinematics director, remembered it well: