After clearing out the abandoned building to get to the street in front of the overpass in "The Bog", Griggs has a Captain Obvious moment regarding the enemy that Lt. Vasquez does not appreciate:
Griggs:[sees that the Marines are surrounded by Al-Asad's forces] SIR, THEY'RE EVERYWHERE! Vasquez: SHUT UP AND KEEP 'EM PINNED DOWN!
The whole door opening exchange in "No Fighting in the War Room!" in 4.
Gaz: Got it, door's coming on line now. [door begins to open... at a snail's pace] Griggs:Oh you gotta be shittin' me! Price: Gaz, can't you make it open faster? Gaz: Negative, sir, but you can try pullin' it if it'll make you feel better. Price: ...Cheeky bastard.
In 2, at the beginning of "Team Player", while covering the bridge-layer, if you look around you can find a Ranger taking cover behind a car with several other Rangers. Unlike the others, he isn't wearing a helmet and isn't firing his gun. What he is wearing is aviators, and what he is doing is eating a chocolate bar in the middle of battle. If you go up the stairs, you can find another Ranger eating a chocolate bar.
While waiting for the airstrike before crossing the bridge, several Rangers pull out their phones to record the airstrike.
The end of The Gulag in MW2 has your team rappelling out of a deep, exploding hole into the sky by a helicopter while everything explodes around you. And then Price of all people has a completely over the top scream of freedom during the epic sequence. Its so out of character that it catches many players off guard into fits of laughter.
In "Heat," Gaz's frustration with the helicopter pilot.
Pilot: Bravo-Six, this is Gryphon Two-Seven. We've just crossed Azerbaijani airspace. E.T.A. is four minutes. Be ready for pickup. Bravo-Six, the LZ is too hot! We cannot land at the farm! I repeat, we CANNOT land at the farm! We're picking up SAM sites all over these mountains! Gaz: Oh, that's just great! Now, where are they gonna land now? Pilot: Bravo-Six, we're getting a lot of enemy radar signatures, we'll try to land closer to the bottom of the hill to avoid a lock-on. Gaz: Oh, he's gotta be takin' the piss! We just busted our arses to get to this LZ, and now they want us to go all the way back down?!
Griggs' first words when he's freed from captivity, along with Price's response:
Griggs: 'Bout damn time. I was starting to think you guys were gonna leave me behind. Price: Well, that was my first thought, but your arse had all the C4.
As a Call-Back to Worm's line in "The Gulag" in Modern Warfare 2:
Soap: Who the bloody hell is Yuri?
When Wallcroft and Burns are advancing up the escalator in "Mind The Gap" a Russian rolls a grenade down the steps. It is pretty easy to avoid, but Wallcroft (who shares a VA with Snark Knight Gaz above) has the most hilariously over-the-top reaction, which doubles as a Call-Back to the first entry.
Wallcroft: CHEEKY BASTARDS!
Pretty much the entire Veteran/Noob commercial for Modern Warfare 3.
In "Wolverines", you can set up the sentry gun on the roof of the restaurant to shoot at Russians that are coming up from an access hatch. However, just like when they rope out of a helicopter, if you shoot them on a ladder they skip the death animation and become limp and weightless ragdolls. If they climb up onto the roof still in the ladder animation when the sentry gun shoots them, there is a good chance that a Russian soldier will be blasted off, cartwheeling hundreds of feet into the air...becoming a little black dot on the horizon.
When the EMP hits the East Coast and aircraft start dropping out of the sky like rain during the "Second Sun" mission, Corporal Dunn succinctly voices what players are probably thinking right then.
Corporal Dunn: HOLY SHIT!!!
A bit of unintentional humor in a mission where you have to protect the Russian President from an assassination attempt. You get him safely to the helicopter, and open the door only to be shot by Makarov, in a fashion similar to how he offed Joseph Allen in MW2. It becomes a bit of a mood disruptor when one starts to half-expect that to happen every time someone goes to open a door.
In the Manhattan Stock Exchange, you see that everyone's stocks are dropping (obviously, since there is a battle in New York going on RIGHT NOW)... except for Infinity Ward, Treyarch, Activision, and Sledgehammer Games.
Here's one from "Persona non Grata". "Yuri! We need to get Soap to the chopper!" *chopper crashes into the hospital room*.
When entering the semi-ruined house on Brookmere Road, one of the first things you will see is a Russian soldier examining the contents of the fridge.
Soap: "We'll stick out like bollocks on a bulldog."
Meta-example: 2011 game of the year award on every game review websites are tense with Battlefield 3 and Modern Warfare 3 just asking for a final showdown we're all waiting for. Gamespot's best shooting game of 2011 award goes to Gears of War 3.
In the "All Ghillied Up" level, if you destroy the helicopter at the Church (which you were not supposed to do—it being a stealth mission and all), Macmillan will sardonically tell you to stop showing off.
Makarov running over a hapless Russian soldier during Yuri's flashback.
During the first level of the trilogy, you are ordered to knife a watermelon.
Gaz: Your fruit killing skills are remarkable!
During the same level, one of Captain Price's quotes upon finishing the obstacle course.
Captain Price: That was an improvement, but it's not hard to improve on garbage.
In "One Shot, One Kill", there's a chance where you can pass by some wild dogs eating a carcass. Letting them be will result in them attacking the enemy soldiers unexpectedly.
Victor Zakhaev's death in Call of Duty 4 becomes unintentionally hilarious if you have "A Bad Year" cheat turned on. You charge towards him, and he shoots himself in slow-motion...and explodes into a pile of tires. Even funnier is Grigg's comment.
Griggs: Kid's got some issues.
In Modern Warfare 2's "Cliffhanger", if you manage to kill steal Soap:
In Call of Duty 4: Remastered's version of "One Shot, One Kill", as a nod to MW3, you can shoot Makarov to get an achievement and/or shoot Yuri for an interesting Easter egg.
Game: Friendly fire will not be tolerated.
If you activate the melon cheat in Call of Duty 4: Remastered, all enemy heads will be replaced with watermelons. Even in cutscenes! It's quite amusing to see Zakhaev glaring at President Al-Fulani up close, his scowl replaced by a big green watermelon.
Furthermore, in "All Ghillied Up," MacMillan's head is replaced with a watermelon too. And he's labeled as "Cpt. MacMellon!"
You can shoot Shepherd in the pit during "S.S.D.D.". You will get a friendly fire game over, but do this in the remastered version yields a trophy... and the same game over.
Game: Precognitive Paranoia trophy obtained.
In the remastered "No Russian", as a nod to MW3, you can now see Yuri. Normally he would be lying down. Throwing a grenade near him makes him immediately run for cover. Killing him yields another Friendly Fire message.