There are two effects that affect hand grenade explosion (and 40mm grenades for underbarrel grenade launchers):

1. class GrenadeExplosion
2. class GrenadeCrater

If grenade explodes on the ground level then they spawn both. As I understand if grenade hit other object (House wall, for example) onlty "class GrenadeExplosion" should spawn.

But for now on 1.66 stable the "class GrenadeCrater" spawns too.

the problem:

1. class GrenadeCrater spawns when 40mm grenade hit walls or other objects
2. It spawns not on the hit position but on the ground.