This happened to me about 6 years ago, at my friend's aunt's cabin. The names in the story have been changed for privacy reasons.

We had all just graduated high school, and Jess suggested that we take a one week trip to her aunt's cabin for some R&R before we had to get to our summer jobs. So altogether, there was me, Jess, her boyfriend Kevin, my boyfriend Chuck, and three other friends, Sherrie, Cory and Lynn. We were also expecting another crew of about 3 friends to show up and visit for a day or two before they headed off to Europe.

Jess's aunt dropped off the key on Tuesday and told us she had not been out there for at least 2 years - not since her husband had died of cancer. It was their place to go together and she said she just can't go back without him, and after this summer the cabin would actually be sold.

We got to the cabin on Wednesday just a little after supper. There were 2 bedrooms and a loft/bedroom. Immediately when we got inside it was very obvious that the cabin had not been visited for a long time, as it was absolutely filthy and a dusty haze hung in the air - we must have kicked up the dust when we went in. One strange thing - there were scratches in the wood floor that looked like some animal made them with its claws. We looked around to be sure that a raccoon hadn't nested in the cabin, and there was none. Nobody seemed to care about the dirt, after all, we were here to rough it and let loose.
Jess, Sherrie, Lynn and I did a little bit of sweeping with an ancient looking broom we found on the porch, and used some of the t.p. we brought just to wipe down the table, chairs, toilet, and other surfaces we would be using a lot. The guys got to building a fire in the wood stove so we could cook up some hot dogs - we were starving.
We finally got all settled in and our bellies were full of hotdog's and salt and vinegar chips. Jess was complaining that she did not feel well, but we all dismissed it as her having eaten too much too fast. She went to lie down - she and Kevin had grabbed the first bedroom off the kitchen, Chuck and I took the second smaller room right beside it, Sherrie and Lynn took the loft, and Cory was to sleep on the sofa (poor guy - it looked like it had been eaten and thrown up on, that sofa). We spent the rest of the evening just messing around and getting situated.
At around 11:30 or so we all hit the hay. I had been asleep for about an hour or so when I suddenly woke for no reason. I had not been dreaming, or at least I don't remember, and I lay there for about 10 minutes before deciding to go pee. Chuck was snoring hard. I tiptoed out of the room and proceeded to the closet-like bathroom directly across from the bedroom. Cory was fast asleep on the sofa as I passed him - he did not snore like Chuck but he drooled twice as bad. I was tending to my business in the bathroom when I heard someone stirring outside the bathroom. "I'll just be a minute...who is it?" I said, thinking someone else needed to use the can. No answer. Still, I could hear someone moving just outside the door. "Jess?" Nothing. "Lynn?" No answer. "Cory?" My voice was a little shaky now because this "person" was not answering me. Suddenly I heard a long, dragging scratch run down the door, like someone had run their fingernails down the wood. I froze. I did not want to open the door, even though I was finished and it was probably Cory playing a prank I thought. I think I was in the bathroom for almost 20 minutes when I heard this "person" shuffle away. I still did not want to leave. I was scared. There was a vent in the bathroom that led to the outside, only about half a foot in diameter. Suddenly the metal grate that was covering it flew off and I looked to see a pale, white, ghostly arm reach into the bathroom from outdoors. I screamed and tore out of the bathroom as fast as fire and immediately I had woken everyone up.
Chuck was the first to come to my rescue - I crashed right into him in the kitchen. Although I was in hysterics I was trying to relay what I had seen, when we all heard the scratching - only this time it was on the roof. Suddenly we heard the most horrible laugh - it was like a classic witch's cackle but it sounded so far away and undertoned with a scream. We all huddled together in fear. Kevin was yelling repeatedly, "What the f@#% was that?! What the f@#% was that?!" Finally the guys decided they were going to be brave and go out there and ambush whoever was playing this horrible joke.
We thought maybe it was the friends we were expecting, only they said they would not be here till Saturday.
Armed with the broom, a knife, and a hatchet, the guys burst out of the cabin and surrounded it. Jess, Sherrie, Lynn and I remained huddled in the kitchen. There was silence for about 3 minutes, and then a short commotion on the roof. The guys returned - their eyes wide and their faces pale. "What?! What did you see Kevin?! Kevin?!" Jess demanded. Chuck was running around moving furniture in front of the door and windows. "What is going on Chuck?!" I demanded. Cory was looking in the kitchen for more knives. He found a bunch and started passing them out to us all. "Someone tell me what the hell just happened!" I yelled. Chuck wrapped his arms around me and everyone was coaxed to sit down while he started to speak.

When the guys ran outside to catch the prankster, what they encountered horrified them. On the roof was a twisted lady. That was the only way they could describe it. She was naked, pale, and she was grotesquely deformed - her head was on backwards; her spine was hunched and twisted; her arms were bent in an awkward position above her head, and one leg was longer than the other but they both appeared to be on backwards as well. They said she had long, greasy looking black hair that dragged on the ground, and her eyes were just 2 black holes with tiny red lights in them. Her mouth was agape like she was moaning, only this appeared to be its permanent state. When she saw them, she scrambled down one side of the roof, and they heard the bushes rustle as she darted away into the woods.

We left the cabin the next day, and were sure to tell our other friends not to bother going out there, because we weren't there... something else was.