The authors report on a 7-year-old boy who suffered an unintentional penetrating cardiac injury while misusing a lawn toy in a predictable way. The boy was hemodynamically stable, with abdominal pain and tenderness. The pericardial effusion was first noted on upper abdominal computerized tomographic images and was confirmed by echocardiography. A puncture wound of the anterior right ventricle was found. As in this case, children may not have the typical signs and symptoms of cardiac tamponade, making the diagnosis difficult and delaying appropriate care. This child's injury could have been prevented through increased parental supervision or a modified toy design that takes into account predictable patterns of missuse.