In the scene where you get on the elevator in Vorkuta, if you look at Reznov's left hand he is missing his index finger. This is because he lost it in Call of Duty: World at War (2008).
The "Numbers" shown throughout the game are shown as Nixie tubes, a neon digital display technology from the 1960s.
Within 24 hours of going on sale, sold more than 5.6 million copies in the US and UK, breaking the record set by its predecessor Call of Duty: Modern Warfare 2 (2009) and making it the largest entertainment launch in history.
Frank Woods actually appears in Call of Duty: Modern Warfare 2 (2009) as a soldier in the beginning of the game.
Agent Hudson is commonly referred to as "Ice Cube" by Mason due to his cold demeanor and indifference. Joseph Bowman, another member of your squad, is voiced by Ice Cube.