Dead cows

[Stan]

In Call of Duty 4: Modern Warfare, the level "Heat" and "Safehouse" features several dead cows in the fields near the barn.
This is said to be a running gag throughout the Call of Duty series, this also being featured in Call of Duty: World at War, Call of Duty: Modern Warfare 2, Call of Duty: Black Ops, and Call of Duty: Modern Warfare 3.
As an addition, this influenced the game's developers to jokingly put "No cows were harmed in the making of this game" in the credits of the first two games

(Proof: http://oi48.tinypic.com/no7mdd.jpg
"No cows were harmed in the making of this game": http://oi49.tinypic.com/97466a.jpg)