It is argued that the “slippery slope” that could be created after the legalization of euthanasia and assisted suicide could open the door to euthanasia practices that go beyond those permitted by law. In this respect, we address three specific circumstances in which we believe this could occur: 1) euthanasia techniques could be applied in non-terminal psychiatric patients and likewise in those with mental disorders or intellectual disabilities; 2) it could also be carried out in adolescents, children and neonates; and 3) involuntary euthanasia may be performed. We provide specific data on these three situations that support the conclusion that all three are already taking place. At the same time, we conduct an ethical assessment of these practices, which we consider illicit.