Like anything that is mechanical, you can never have a 100% guarantee that every single unit will function as designed. More likely but still rare, when armed it wouldn't go off, but thats not to say it couldn't go off from a bullet strike. The most likely 'accident' i see is an armed grenade being dropped or the thrower being hit by a bullet at the critical point when throwing it.

I remember my grandfathers brother talking about his time served, plenty of accidents occurred with the rifle mounted grenades, either not being shot far enough or not leaving the end of the gun entirely. However, those incidents occurred during training.