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.

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.

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.