It is a reasonable assertion that the biggest injustice that can happen within criminal law is to be accused of something which you are completely innocent of.
This can occur in different circumstances with either the police suspecting you of committing a crime and arresting you, or someone falsely, and possibly even maliciously, accusing you of a serious crime, which the police must act upon.
Stories of miscarriages of justice where innocent individuals have has their life destroyed and spent many years in prison can be extremely scary if you are innocent and being accused.