There was a zombie apocalypse going around town, there was only a few survivors left, lucky.. You trained yourself on how to aim well, so your aiming is pretty much of a 'perfectionist'.

While you were walking around, trying to be quiet as possible so the zombies won't hear you.. Someone bumped into you from behind, you thought it was a zombie, so you quickly turned around and took defense, until you realized it's a human. You didn't believe if that was a real human or a zombie in disguise