Call of Duty: Modern Warfare Remastered

In the Mission, "One Shot, One Kill," you can see the Modern Warfare 3 villain Makarov in one of the cars. If you shoot Makarov after you shoot Zakhaev, you can unlock the achievement "Time Paradox" and complete a secret objective where you have to change the future.

At the UK warehouse in "FNG", the exit sign above the door is the U.S. version, not the British one. The British one is green with a running man, whereas the U.S. one simply says "EXIT".