These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Mark Of The Ninja
Goddamned Bats: The Dogs. They can smell the ninja even in the darkness and will start barking alerting the entire and room giving your position even if you try to move to another spot.
Demonic Spiders: The Stalkers. Not only they cannot be stealth killed as they have keen senses (they can identify the ninja within a certain radius), are extremely agile, persistent, cunning and their attacks can kill the ninja instantly. They also need to be stunned akin the Elites but are far more agile and acrobatic meaning that physical combat will most likely result in a No Sell.
Fridge Brilliance: Use a Flare Bomb against enemies wearing Night Visions Goggles and you'll blind them.
Player Punch: Ora isn't real? And you want me to kill her? Even if she really is just a hallucination, Ora has been the player's guiding voice throughout the whole game.
And if you do kill her, she dissipates into a flock of crows, and the scene changes to the Marked Ninja impaled on his own sword, having committed seppuku.
When you begin to make your decision, but before the execution, your target will be shocked, then lower his or her head in a combination of sorrow, disappointment, shame and acceptance of his or her final fate. Hesitate and it's possible to change your mind, but the individual remains in the same position. It prevents any real clear 'right' decision, as both characters will accept and respect your final judgment.