A Catholic priest cannot reveal what someone has told him in confession. If he does reveal it, he will be defrocked and punished.
If a different Catholic than the priest unintentionally overheard the confession, and learned of something important, like a murder. Would the bystander be allowed to reveal what he had heard? Does the answer change if the bystander is also a priest?
Of course, a priest who received a confession of a murder would recommend that the person confessing should give himself up to the police. But assuming he refuses to do that, what would be expected / permissible?
This is a hypothetical question, but in the centuries since the seal of confession has existed, this must have come up.