At one point, I would have dismissed the idea of demonic possession, but I have seen enough that I do believe it exists.
Two of the kids I taught had been friends for many years. One of the boys had a real interest in the occult, the other, was just a very sweet, confused child. My aid and I both noticed that on days that one would be relatively normal, the other would exhibit some really odd, aggressive behaviors. The most dramatic episode occured with the sweet kid curled up in a fetal position, chanting words or sounds I could not identify, alternating in a small child's voice, and a gutteral male voice. The aid brought her Bible with her and layed it quietly on her desk. At that point the kid, whose back was turned to her, went ballistic. I was speechless.
I don't think the two boys were playing a "hey, I'll be bad today, you be bad tomorrow" game. I don't think the kids were even aware of the pattern. It certainly gave me food for thought. It was almost like "something" would jump from one of them to the other. I always wondered if the other kid had done some experimenting and had something attached to him that also for some reason, attached itself to the other child.
A similar thing happened to my husband. After graduating college, he tried his hand at teaching home-bound children, who either for medical or behavioral reasons could not go to regular schools. He was assigned to a little girl, about 6 years old. To make a long story short, this child exhibited some pretty bizarre behaviors. One day, she was having a particularly bad episode, and James was sitting there, silently thinking to himself, "Okay kid, if you really are possessed, go over there and pick up those tennis shoes on the floor..." The little girl quietly got up, walked to the shoes, picked them up, brought them to him, and smiled the most sinister smile he had ever seen. Had I been there, I would have made a new opening in that house.
I think for the most part, diagnoses of mental illnesses are accurate. There are just those few that leave some doubts in your mind.