The decision not to prosecute the individual alleged to have raped Jade McCrossen-Nethercott in London, 2017, is the result of the uncertainties that surround the sleep condition, sexsomnia (a parasomnia in which a person engages in sexual activity, either solo or with a bed partner, while asleep). Anchored in McCrossen-Nethercott’s case, this chapter elucidates medico-legal controversies surrounding sexsomnia, which are here coupled with the systemic failure of the criminal justice system to grapple with issues of the reliability of expert evidence. The sexsomnia defence has a turbulent history and was the focus of a heated controversy about whether sleepwalking following intoxication could be used as the basis of an automatism defence, raising an additional question of whether sleep medicine was able to adequately discriminate between sleepwalking and alcoholic blackout to serve as a forensic tool. However, the clinical tests for parasomnias, and the medical knowledge upon which they are based, are highly contested and ultimately provide such a low-threshold for diagnosis that many persons without a recognised sleep condition would be found to suffer from parasomnias. Is there a risk that claiming a person was in a sexsomnia state and hence could not recall sexual activity is a very easy allegation for a defendant to make and difficult for the prosecution to discount as a possibility? To this end, this chapter discusses McCrossen-Nethercott’s case specifically in the light of the alcohol-induced sleepwalking controversy and, in particular, asks questions about the admissibility of expert evidence and the proper application of the burden of proof in such cases.