So you come around a corner and an enemy player is kneeling there.  You are 90 degrees to his immediate left.  He looks oblivious to your presence, his avatar is even comically waiving you on, so you move in for the knife to not give your position away to the rest of his team.  Suddenly he opens fire on you and you die in a spray of bullets coming from a weapon aimed well to your left.  I guess the moral of the story, just pull the trigger and keep moving.