A Ghost in the System is a bit of loose programming that developers overlook in a game that makes something strange happen. This has been observed in games most usually like Call of Duty, Halo, and Battlefield. These have been around since the first video game ever- pong.
It is expected that all games have at least one or two in them but it has not been proven true on every single game so far. They are most common in First Person Shooter games and least common in arcade games. Not all of them are discreet either.
Here are a few examples of Ghost in the System: The Ghost of Lockout/Blackout myth in halo, Teleporting Sniper in Battlefield, Ghost Car in pretty much every racing game ever- there are two examples of this, one being when there's a car nobody seems to be driving in the game, the other where there's an invisible car in the game.