In 2008, Dr. Richard E. Gallagher, a board-certified psychiatrist and associate professor of clinical psychiatry at New York Medical College, documented the case of a patient nicknamed "Julia" whom he deduced was indeed possessed by demons. It's rare that a scientist and psychiatrist would acknowledge the possibility of possession; typically doctors think that possession is either fraudulent or a result of mental illness.
Dr. Gallagher personally observed items flying around the room, Julia levitating off the bed, speaking in tongues, and knowing things about people around her that she could not possibly have known. Here is an excerpt from Gallagher's statement:
“Periodically, in our presence, Julia would go into a trance state of a recurring nature. Mentally troubled individuals often ‘dissociate,' but Julia's trances were accompanied by an unusual phenomenon: Out of her mouth would come various threats, taunts and scatological language, phrases like ‘Leave her alone, you idiot,' ‘She's ours,' ‘Leave, you imbecile priest,' or just ‘Leave.' The tone of this voice differed markedly from Julia's own, and it varied, sometimes sounding guttural and vaguely masculine, at other points high pitched. Most of her comments during these ‘trances,' or at the subsequent exorcisms, displayed a marked contempt for anything religious or sacred.”