The throwing knife of instant death
This happens with arrows too, and pretty much any penetrative projectile weapon, but is especially bad with throwing knives. A character will throw a knife and it lands with perfect accuracy, tip-first, into the enemy, causing them to instantly drop dead. As long as the knife hits anywhere in the torso (except the shoulders of course) or the head, it hits an off switch. Doesn't matter how deep the knife actually penetrated, it will stick straight out of the guy and stay imbedded. If the guy was charging at the thrower, they'll whip back and their legs will fly forward like they just got clotheslined.
And they do this with blade weapons that aren't made for throwing, meaning the balance is off, and it still hits tip-first. Even if the character is an expert knife thrower, that still doesn't mean the enemy will instantly die from a hit. Thrown objects lose a lot of energy flying through the air, especially if they're not balanced for flight. Then they have to get through clothing. Most likely it's just a flesh wound. Let's say it goes deep enough to penetrate a vital organ, the bad guy would be rolling on the ground in agony bleeding out for a while.
The only way this trope can be make logical sense is if the knife had struck the base of the brain (medulla) or the bad guy was in so much psychological shock upon being hit that they just freeze and play dead.