An L-train was planned to be used as an interactive element in at least one multiplayer map. Scripting and a kill icon can still be found in the games code, however, none of the released maps feature a subway system, so it's unknown where this might have been used.

The script reveals that it would have been a 24-car train, causing rumble effects, moving at 1200 units/second every 100-200 seconds, shaking the screen of anybody standing nearby and killing anyone standing in its path.

In the earlier versions of the mission Cliffhanger, Spoiler:Captain Price was meant to be with the player instead of Soap, and [spoiler]the player was meant to play as Soap instead of Roach.

The cutscene that Spoiler:briefly takes the player into space as an astronaut, before being killed by the ICBM in the mission Second Sun, was going to be an interactive level, similar to that of "The Coup" in the original Modern Warfare. This was later scrapped.

On both the singleplayer mission "No Russian" and the multiplayer map "Terminal", by using noclip or spectator mode to go to the shops on the second floor you can find two floating game cases of Call of Duty 4: Modern Warfare in the liquor store.

In the Russian version of the game by 1C Company, the No Russian mission is not available for play. In Japan and Germany, if the player kills a civilian, the game will restart from the last checkpoint with the "You killed a civilian!" message. In the Japanese version, Makarov's line, "Remember, no Russian" is mistranslated as "Kill them, they are Russians." This was criticized as the plot of the game was centered around the terrorist's motivations and as a result, the mistake caused some confusion.

In the Spec Ops missions "Suspension" and "Wreckage", the cargo ships on the river below the bridge are the same as the ship from the mission "Crew Expendable" and multiplayer map "Wet Work" from Call of Duty 4: Modern Warfare.

Judging by an unused file in the game data, the mission "No Russian" was originally going to take place in a real-life location: Domodedovo Int'l Airport in Moscow. It was probably changed to a fictional one in order to avoid further controversy.

During Mission 5, Takedown, Ghost is seen preparing a pair of jumper wires to torture Rojas's Assistant. During the same scene in the game's trailer, a random soldier is preparing the jumper cables instead, and the man in the chair has a sack on his head.

There are multiple cut game modes, as revealed by the existence of sound files for them within the game's data. Some of these modes can even be played by hacking. These include:

• Arena: A game mode with no respawning. The objective was to kill all of the players on the other team, but if there was only one person left alive on each team, a flag would randomly spawn and the first person to capture the flag wins.
• Arms Race: Unknown. Only sound files remain.
• Defcon: There would be 5 Defcon phases which would count down. Play is normal at five. At four, each player goes into "Last Stand" upon death. Three makes weapons deal more damage. Two reduces the requirements for a kill streak by one kill. Defcon one disables respawning. After another minute, if one side has not eliminated the other, a nuke detonates, bringing the game to a draw.
• Die Hard: Team Deathmatch with Last Stand available for all players.
• Global Thermonuclear War: Involved protecting a nuke from the enemy team until the match was over.
• One Flag: One team protects the flag, while the other team captures it.
• VIP: Like Capture The Flag, except protecting a random player on the same team.

Unused coding and sound files remaining in the game show that at one time the MH-6 Little Bird was to be a pilotable killstreak reward, instead of just delivering care packages and appearing in the campaign.

It can be restored through modding the game, but is hard to pilot, and will clip through the terrain.

The achievement/trophy for killing a Juggernaut in any Spec Ops mission, "I'm the Juggernaut...", is a reference to a popular online video of somebody dubbing over The Juggernaut from the X-Men cartoon series, stating "I'm the Juggernaut, bitch!"

In the "Favela" level, some players complained about the Arabic script on the picture frame that is written "Allah is beautiful and He loves beauty" in the bathroom.

The map was removed from the game on the 6th October 2012. It was put back into rotation on the PS3 version on the 23rd of October after an update which replaced the frame borders, and on December 10, 2012 for the Xbox 360 version.

Unused dialogue for "Of Their Own Accord" suggested that if the player neglected to, or otherwise failed to defend the Washington Monument evac site, it would eventually be overrun and destroyed by the Russian army resulting in the deaths of all military personnel and civilians on-site, and thus mission failure.

The dialogue itself features the previous operator being replaced with a far more desperate one warning the player of the site's worsening combat effeciency. This would eventually result in the untrained civilians taking up arms against the Russian invaders before being completely overrun. In the original final game, effectiveness never goes below 80% and the danger is more to the player themselves.

The 2020 campaign remaster implements some degree of dialogue, including combat effectiveness dropping to 50% if the player is on harder difficulty levels, though never lower than that.