This one's easy. It was in Battlestar Galactica when....
after a successful mission Boomer goes up to the bridge and is welcomed cheered by one and all. Then, Adama reaches out with his hand to offer a congratulatory hand shake. As Boomer walks over, smiling from ear to ear, she begins to reach out towards Adama's hand to complete the gesture. It is at this point that, almost too quickly for it to register, you see that she is in fact holding a gun in that hand, with which she proceeds to shoot Adama in the chest. General panic and disbelief fill the room. I literally said, "Damn! What the hell just happened?"