There was an incident in the early 70s where a reserve qualifier lost a grenade and it had gone into the safety pit. The qualifier realized it and tried to exit and the safety office pulled him back in. 69th Infantry Brigade. I was in the support battalion and one of our companies was the Medical Company that had to respond.
If I remember my pit construction rules from back then (1977), there is a grenade hole in the pit that the grenade should roll into or be pushed into. This hole allows the grenade to explode with less possible damage to the people in the pit, and keeps them from jumping out of the pit when there might be more explosions or enemy fire occurring.