Extending that principle, if a suspect accidentally hands over the murder weapon, with his finger prints all over it, it should be returned to him so he can destroy it. What am I missing?
The difference between physical evidence in a criminal case, and a document made confidential by law in a civil case. If an attorney, or his/her secretary accidentally hits "CC all parties to this case" when trying to email a confidential document to one co-defendant (or co-plaintiff), it doesn't mean that the other side gets to introduce the confidential document at trial.
Of course they now know about it, and can try to get the document from another source, so that it can be admitted.