This is not my experience; it happened to an acquaintance of my significant other. This girl (I'll call her Karen) was home on leave from the armed forces and went to visit old friends who formed a get-together in Karen's honor. After a while, a few people left, leaving only the owner of the house (Mary), Karen and another friend (Vicky).
At one point, Mary went to get something in another room, and Vicky stepped into the bathroom. When they returned to the living room, Karen was not there and they both wondered where she'd gone. They called out her name, but got no response, so they agreed to go in separate directions to look for Karen.
Mary went out to the backyard, and it wasn't long before she found Karen sitting on a stair-step, hunched forward, with her elbows resting on her knees and her hands covering her face; she was also rocking back and forth. Mary, agitated, kept asking, "Karen, are you OK!!!??? Karen, what's wrong!!!???" But Karen was not responding...she just kept rocking with her hands over her face. Mary tried a couple of times to remove Karen's hands, but she was holding on with tremendous force. Finally, Mary was able to free Karen's hands somewhat, and when she looked at her face, she saw something she'll never forget! Mary said it was not Karen's face! It was the face of something very, very evil! At that point she yelled out for Vicky (who can "see things"), and Vicky came quickly. They managed to drag Karen inside, where she slowly began to come around after Vicky tried to oust the entity. They removed Karen's clothes and burned them, hoping to get rid of any traces of the evil that had invaded her body. Vicky told Mary that she felt something extremely powerful and dark when she reached Karen, who in turn said that she remembered a very tall, dark man all dressed in black, saying to her, "I'm gonna get you!!! I'm gonna get you!!!", and she tried to get away but the dark figure caught up with her.
Mary told Karen to stay over, and Vicky decided to stay, too. Later that night, Karen was sitting on the couch watching TV. She looked up at the ceiling fan and started screaming her head off! She said, "My God, he's back!!! He's up on the ceiling! He wants me!" Vicky looked up and confirmed what Karen was seeing, although Mary saw nothing. Vicky said it was the scariest thing she had ever experienced, but she acted quickly to get rid of this evil presence.
Very fortunately, nothing else happened after that; apparently Vicky did her thing well! I am no expert on these things by any means, but I have read and heard that evil spirits and demons attempt to take over people's bodies when the person is in a weak state, caused by a myriad of reasons. In this case, although Karen was happy and eager to go home, she was suffering from a lot of stress from the armed forces. This certainly is a weakened condition in a person, and perhaps the fact that Karen was dressed all in black had something to do with it. Who knows?