In the days before x-rays, a nail in the skull was a popular homicide technique

The Method of the Nail

Apparently the 'method of the nail' used to be quite a popular homicide technique, because in the days before x-rays it was hard to tell that someone had a nail in their head. The victim's hair might hide the wound, so people, not seeing any obvious sign of injury or foul play, would often assume death occurred from natural causes.