Here is the text of an Interagency Aviation Safety Alert that was issued on March 23, 2010 by the U.S. Forest Service. It describes damage to a helicopter rotor blade when someone attempted to throw a knife from inside the helicopter to someone standing nearby. Click on it to see a larger version.
The safety alert goes on to explain that a similar incident happened a few years ago when a second rocket scientist did the same thing with a set of keys. The document ends with this: