One Halloween while in the Army, we set up our Company area as a Haunted House for the soldiers in our unit to bring their families to. Me and a friend named Flood set up the Med Station as a mad scientist lab; I was dressed up like Claud Rains in the Invisible Man, and Flood was wearing cheap prosthetics to look like he'd been hacked up.
So, our idea was to have him on a gurney and I'd stand in front of him with my back to the door, and when you walked in all you would see is me hunched over him and it looked like I was doing surgery, but you couldn't see his "wounds." We had one of his arms slung up and hanging from one of those overhead traction things, and taking a cue from Savini's Grande Illusions book we rigged up a trick needle so that when I touched his arm with it and pressed the plunger, it'd streak fake blood and he'd moan and groan and whatnot. Then I'd turn towards you and Flood would sit up really fast and scream while reaching for you. That was the idea. Only thing was, I didn't take into consideration that with my back to the door, I wouldn't be able to see who was coming in...but I'll get to that in a second.
So we set it up, went over the drill a few times, practiced it. Then they tell us the first group of people are coming through. So, I turn my back to the door, I'm all hunched over him. We hear the door open.
I put the rigged needle against his arm and press the plunger. Fake blood streamed out; worked like a charm. He moaned, groaned, "Help, he's killing me!"
Then when I figure the people who just came in are close enough, I spin out of the way, Flood sits up, screams...
That wasn't Flood; that was our Company Commander's 7 YEAR OLD DAUGHTER. We scared the living piss out of her. He scooped her up, carried her out, she's screaming the whole way, "Daddy, Daddy, Daddy, AAAAAHHHH!"
The next day, he chewed our asses and forbade us to ever participate in the company haunted house again.