Unfortunately, this is very possible. A 40mm grenade cartridge consists of a short aluminum cartridge case and the HE grenade. The 40mm grenade is notorious for producing very unstable and dangerous duds, taking very little disturbance to set them off. After the grenade cartridge is fired, the cartridge is indeed "spent", but if the grenade didn't go off when it hit the groundor the target, it becomes a very dangerous dud, because the safety and fuze (partially) have already been activated.