The Sixth Amendment to the U.S. Constitution guarantees a criminal defendant's right to "be confronted with the witnesses against him" in all prosecutions. That's one reason why the state can't bring hearsay evidence against you in a criminal case, for example. You have the right to see, hear and confront any witness (or even a document) accusing you of any crime. It's an essential part of due process of law.
A 19 year-old suspected of calling in bomb threats to dozens of Jewish community centers in the U.S. has been arrested in Israel. The man, a dual U.S.-Israeli citizen, has put forward a pretty compelling defense: a brain tumor made him do it.