In Super Mario Bros. there are ghosts. You look at the ghosts, they don't move.
This Call of Duty: Black Ops 3 easter egg is like that. Only actually terrifying!
The video above does a great job of explaining why this easter egg is genius, but I'll give a quick recap.
In the above multiplayer map there are mannequins. If you shoot the heads off all the mannequins they transform into... ZOMBIE MANNEQUINS. That's pretty awesome in its own right, but already widely known.
But there's a second easter egg. One that's far creepier.
If you shoot the arms off all the mannequins they still come after you like zombies.
They only move when your back in turned. Like some terrifying goddamn nightmare. Like something from the mind of Stanley Kubrick. Goddamn.
Pure horror genius.