I won't really call this scary, because I didn't shit myself, anyways:
Counter strike source zombie mod.
The T's and CT's are allies in this mod. About 30 seconds into the game, one random person is chosen as a zombie, their skin looks like the zombies from half life 2. Everytime the zombie stabs a human, they become a zombie. Anyways:
Me and 4 other people were ontop of this mountain (Let me tell you, the zombies can stack on eachother to reach you). There was a horde of zombies below us trying to stack on eachother, we had to shoot the ones ontop so they couldn't stack, the map we were playing on was VERY foggy. Then, all the zombies walked away for some reason (Note, these are real players, not bots) after the last survivor on the otherside of the map was killed, all the zombies came to us at once. It was fucking scary, they came from all directions AT ONCE!.
We then died and the zombies won the round.