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Pre-sentence psychiatric expertise is currently in a paradoxical situation, regularly called into question by the media and society, while its missions continue to grow. The criticisms can be reinforced by the divergences between experts, which seem to testify to the sometimes-controversial character of psychiatric knowledge.The aim of this study is to identify the clinical situations most likely to give rise to disputes in terms of medico-legal responsibility. It was carried out with 58 patients admitted to the Henri Colin Unit for Difficult Patients (UMD) following a homicidal act, over a ten-year period (2008–2017). The results show four situations of divergence: presence of psychopathic personality types seeming to predominate at the time of the act; diagnostic uncertainties between persistent delusional disorder of the paranoid type and paranoid defenses; presence of a schizophrenic disorder of the hebephrenic type; and finally presence of a psychotic breakthrough with perverse and narcissistic personality traits. The recruitment of patients in a hospital setting introduced several biases to the study.Further research on specific populations or in prisons is necessary, as is opening up the reflection to expert practices abroad.