The case of Daniel M'Naghten is debated in the House of Lords soon after he is acquitted of murder by the British court. His case establishes the M'Naghten rule, that individuals whose mental illness render them incapable of judging right from wrong are not guilty of a crime, on grounds of insanity.
Queen Victoria is incensed by the verdict. However, even the prosecutor, solicitor-general Sir William Follett, had to admit at the end that "I cannot press for a verdict against the prisoner..."
All of the medical evidence for insanity and reduced responsibility were on the die of the defense and Follett had nothing to counter with. It's rare that a prosecutor, during a trial, says that the defendant shouldn't be found guilty.