I saw this happen once when we had a patient with severe delusions and hallucinations. He was admitted as a paranoid schizophrenic to the medical psychiatric ward I worked on. He was so out of control he had to be restrained to keep him from hurting himself or others. Only when he fell into a coma and started running a fever, did the questions begin. Turned out this man had encephalitis. He was treated with the appropriate medications and made a full recovery. His behavior blinded everyone to a severe underlying condition. It probably started with a headache, but by the time we got to see him, he couldn't tell us about what led up to him seeing the pink elephants. He was too sick.
Thank goodness I love a mystery. :-)