Suppose a new kind of lie detector is invented and it's about 95% accurate, by allowing the results of this lie detector test to be used as evidence in court more people who are actually guilty will be convicted, however (for the sake of argument) just as many will be falsely convicted.
This is across all forms of crime, from jaywalking to mass-murder, so is it better to ensure the guilty are punished or to avoid punishing the innocent?