The final traumatic event recalled by Anna O. during her treatment with Josef Breuer was a terrifying hallucination she once had of a black snake attacking her ailing father. This event has been variously interpreted as indicating an underlying psychodynamic conflict, as a temporal lobe seizure, and as an hypnotic confabulation. We argue, however, that the hallucination--during which Anna O.'s arm was reportedly "asleep" due to nerve blockage--was probably a sleep paralysis nightmare. Sleep paralysis nightmares continue to be overlooked or misdiagnosed in clinical practice, and, in recent years, have been implicated in the controversy surrounding memories of trauma and sexual abuse.