The physics engine in Halo: CE can be really silly at times.
An enemy hit with a vehicle will result in a One-Hit Kill, no matter how tough that enemy is or how fast the vehicle is travelling. This can lead to hilarious situations such as slightly nudging Hunters with the lightweight Ghost and instantly killing them.
One time I was crossing one of the bridges on Assault on the Control Room, and I got to the part where there's an Elite Zealot on the other side of a short wall. I toss a grenade over the wall and The Elite dodges sideways, diving behind a Shade turret in the corner. The grenade goes off and sends the Shade barreling through the Elite (squashing him flat) and falling over the side.
Sergeant Johnson's inspiring pep talk to the marines at the beginning cutscene of the first game, and all the variants therein:
Normal: "When we meet the enemy on the ground, we will rip their heads from their spines, and toss them away, laughing!"
Legendary: "I don't care if it's God's own anti-son-of-a-bitch machine or a giant hula hoop! WE AIN'T GONNA LET 'EM HAVE IT!!! What we will let them have, is a belly full of lead, and their own blood to drown in!!"
"Am I right, marines?"
"SIR, YES, SIR!
"Mmm hmm. Damn right I am!"
Upon freeing several imprisoned marines alongside Captain Keyes, who reveals information about what Halo is that he overheard from his stay in prison, Keyes gives out orders to find Halo's control room while the rest listen silently before moving out. The chapter name, however:
Speaking of section titles, many of them are funny:
AI Constructs and Cyborgs First! Wait, It Gets Worse! But I Don't Want to Ride the Elevator! Fourth Floor: Tools, Guns, Keys To Super Weapons Breaking Stuff to Look Tough …And the Horse You Rode In On Warning: Hitchhikers May Be Escaping Convicts
During the intro cutscene of The Two Betrayals we get Cortana's enraged entry upon throwing Guilty Spark's "procedure" off the rails.
Cortana: I've spent the last 12 hours cooped up in here watching you help that thing to slit our throats!!
Chief: Hold on now, he's a friend.
Cortana: (sarcastically) Oh, I didn't know! So is he your pal, your chum? (Serious) Do you have any idea what that Bastard almost made you do!!??
Chief: Ye-e-e-s, activate the Halo's defence systems using the index, which is why we brought it here.
Cortana: (holding the index) You mean this?
We then get this exchange between 343 Guilty Spark and Cortana in which the latter's effort to throw the former's "procedure" off the rails sends him into a fit that actually breaks his -until now- unbroken expositionary demeanour.
Guilty Spark: A Construct? in the control room? That is absolutely unacceptable!
Cortana: SOD OFF!!!
Guilty Spark: (in a fit) What impedance! I should purge you at once!
Cortana: You think that's a good idea? (the index then vanishes from her hands)
Guilty Spark: (in fury) Wha... How... HOW DARE YOU!!!!AHH!! AHGG!!
Cortana: Do what? I have the index, you can just float and sputter!
The cutscene before the level Keyes in the original game was funny; Cortana thinks she's figured out how to Teleport the Chief. She manages to do it, but she mistakes the ceiling of the ship with the floor.
Immediately afterwards when that mistake finally exhausts the Chief's patience and he Dope Slaps his own helmet to shut her up.
Cortana: "Oh now I see, coordinate data needs to be" * donk* "Right...Sorry?"
In the level The Maw (the last level in the game), Cortana's trying to tell the Chief to pull up on his hijacked (and heavily damaged) Banshee or else they'll crash, while Chief casually disregards this. They end up crashing, the Chief none worse for wear, and Cortana betting that he did that on purpose.
Cortana (panicky): "This thing is falling apart!"
Chief (calm): "It'll hold."
Cortana: "We're not gonna make it!"
Chief: "We'll make it."
Cortana: "Pull up! PULL UP!!" *CRASH!* "...You did that on purpose, didn't you?"
After 343 Guilty Spark has shut down the self-destruct of the Pillar of Autumn, Cortana is at a loss as to how to blow the ship up. The Master Chief asks how much firepower it would take to crack one of the engine shields. Cortana replies that it would only take a well-placed grenade. Cut to the Master Chief tossing one in the air when she asks why.
Anything Grunts say. Sometimes you'll walk into a room and randomly hear screams of fear, even though they don't know you're there yet. Also:
when you've returned to the Pillar of Autumn and go through the dark vents again, just outside your exit is a battle between Covenant and Flood. But because of the narrow doorway, you can mostly only see the bullets and grenades flying back and forth, and hear the NPCs' reactions offscreen, turning an otherwise-ordinary battle into hilarity.
Shooting at 343 Guilty Spark in The Library in will generate a variety of responses, the best being "Stop being human!"
The Legendary ending to the first game. "This is it, baby... Hold me." Johnson and an Elite hug, and the Elite grabs Johnson's butt. In Anniversary, they still hug, but Johnson grabs the Elite's butt. The Elite is briefly shocked before the Pillar of Autumn explodes.