Here's a moral dilemma. The unthinkable happens -- some awful person rapes your 13 year-old daughter. Unfortunately, it's not possible to convict him, so you move away and try to put your life back together. A couple years later, you're at a restaurant when your favorite show comes on TV. At the same time, you notice the rapist sitting at another table looking not very happy. You choose not to confront him and try to just watch your show instead.
A few days later, you learn that the rapist has been arrested for murder and that there's a good case against him. You are delighted at the thought that he'll likely go to prison. However, you learn that the time of the murders coincide with the time you saw the rapist in the restaurant. You're sure of the time because of the favorite TV show that was on. There's no way he could be the murderer. Perhaps he hired someone to do it, but you don't know that for a fact.
If you come forward and tell the truth, the rapist won't be wrongfully convicted of murder. If you keep your mouth shut, he'll likely go to prison. What do you do?