In order for someone to be tried in a Federal Court, a Grand Jury must be present at the trial. Additionally, someone cannot be forced to to testify against themselves, they cannot be charged with the same crime more than once and that all citizens must be given due process of the law before conviction.
Example 1: A police officer forced someone to admit fault through threats and increased prison time.
Example 2: Someone is sent to prison after committing a crime, but never went on trial or received due process.
Example 3: A man crashes his car into two other cars. He cannot be charged twice for hitting each individual car because it is the same crime.
