In "F.N.G.", if the player triggers the target at station one and then runs back to the platform where Gaz is standing before shooting at it, Gaz will ask you what you're doing and tell you to get back to the first station. You can also get a wide array of failure messages for many circumstances, including shooting the TV's at the training course ("You endangered your squad!") or shooting too close to Price ("You almost shot Captain Price!").
Price actually has a possible line referencing the fact that about half of the level's objectives were Dummied Out ("You're getting slower. Perhaps it was a mistake to let you skip the obstacle course.")
In "Charlie Don't Surf", it's possible to see an enemy technical truck driving far in the distance as you enter the town. If you shoot slightly ahead of it and view it as long as possible, there's a special animation where the driver's door opens and a corpse rolls out. This cannot be done anywhere else in the game.
In addition, if you take out that technical, the one that attacks you on the road won't appear, with the implication being that it was the same vehicle.
Using cheats on "Game Over" will produce a wide variety of unique effects that can't be triggered in any other way. If the player activates the "godmode" cheat and scores a hit on the Hind chasing the truck with an RPG (which is next to impossible) or kills the maximum number of enemies on the bridge before the tanker explodes (which is very difficult), the ending changes - instead of Zakhaev executing Gaz, a random SAS soldier (usually named Pvt. Cook) inexplicably gets up from the dead and melees all three Ultranationalist soldiers (including Zakhaev), with the rest of the ending proceeding normally and the player not having anyone to shoot.
If the player uses the "noclip" command to move around while they're laying on the ground, Zakhaev will follow your movements and unload his gun into you. Also, if you use "noclip" as Kamarov and the Loyalists arrive to rescue you, you can watch as the medic continues trying to resuscitate Price after the stretcher is lifted up into the air, and the medic pounding on his chest at the end (which can't be seen in normal play).
In "Blackout", if you follow Gaz to the fusebox instead of waiting by the door with Price, he'll tell you to go back and wait.
During the mission before handling the nuclear bomb launch crisis, it is possible to be killed by a hanger door opening by jumping on it before it fully opens, complete with the message "You were killed by the hanger door"
In Modern Warfare 2:
If you wait long enough at the basketball court during "S.S.D.D", a Ranger will ask you to get off the court. If you wait even longer, he asks you to wait your turn, then expresses indignation at what you're doing.
If the player decides to run across the completed bridge after it's been repaired in "Team Player", you get shot by a sniper and the game chides you with a special message that "Maybe it's safer to stay with the convoy." Later on at the end of the mission, if you shoot General Shepard in the arm or the leg before he begins speaking to you (which happens almost instantly), he adopts a battle stance and looks at you in disgust, which makes the mission impossible to beat.
If you decide to glitch out of the level and visit the road north of the school (which is impossible to access normally), you can see a Spetznaz flag flying on top of the school - the only time in the game any flag is seen in such a fashion.
At the end of "Contingency", if you decide to follow Price towards the sub instead of taking position on the rooftops to protect him, he will tell you to get back to your assigned place. That's not the "dev" part, though. By using the "noclip" command, you can follow him inside the sub and watch as he goes through the whole procedure of arming the nuclear warhead and firing it! This is due to a section of the level being cut from the final game.
The metal detectors in "No Russian" are not just a one-time scripted event. Mess around with the ones in the multiplayer version of the map, "Terminal", and you will notice that, not only do they go off if you run through them, but throwing any lethal equipment or launching a rifle grenade through them also sets them off.
In the Modern Warfare 3 mission "Turbulence", if the player attempts to reload the AK-74u or Five-Seven as the plane stalls and floats in zero-G gravity for a few moments, the magazine will float away and spin in the air instead of being thrown away normally. This cannot be done with any other weapon.
Near the end of the second-act level "Mind The Gap", you can shoot and kill the detained terrorists as you rush to leave the station. Doing so will cause the cops who were arresting them to stand up and look on like nothing happened.