I don't know if I'm late to the party, but one of the creepiest stories I've ever heard actually is a real story. So this happened to a very close friend of mine. We shall call him Josh. So Josh and his family frequently visit his grandparents, and when they do they stay overnight. Many years ago on one of these trips, Josh got the privilege to have his own room, even though he was fairly young. He stayed in a guest room that was quite large for his needs, fully furnished with everything from a wardrobe to a large mirror on wall. Anyway, bedtime rolls around, and Josh drifts off to sleep.
For no apparent reason, Josh woke up in the middle of the night. He turned to check the time on the alarm clock near his bed, and recalled it being around 3am. Then sleepily taking a last look around the room his gaze falls on the mirror. He sees himself in the mirror, but also someone else, standing behind the headboard of his bed. He described him as a medium build hispanic man, with a plaid t-shirt and a hat. Obviously Josh at this point is scared out of his mind. I don't know what I would have done in his situation, but Josh said he pulled covers over his head, and freaked out for a couple hours and fell back asleep, trying to forget it.
Six years later, he inquires about it with his family. His grandparents and parents tell him that a long time ago there was a man who died of a heart attack in the bed Josh was sleeping in (I'm assuming he means the room not the exact bed). Josh looks at some pictures of the guy, and it turns out to be the same guy he saw in the mirror. Remember, Josh had never seen or heard of this guy at all at the time six years ago when he saw him in the mirror.
Tl;DR Friend looked in a mirror and saw a strange man standing behind his bed. Finds out years later from family that a man had a heart attack and died in that room. Friend looks at pictures of guy, same guy as in mirror.