Trying to throw away a live frag grenade has a very high chance to bug out, maybe 100% in the current build of the game. Instead of throwing the live grenade away, it becomes stuck in your character's hand, and you blow up. If you try to throw away a smoke grenade before it is fully activated, you may have your inventory freeze and become unable to switch to any of your weapons until you pick someone else's dropped weapon up off the ground.
Here is a video of the frag grenade throwback bug in effect:
I have encountered these bugs 2 more times since I recorded this footage less than 1 week ago, so I won't even attempt to use the throwback mechanic again until I know it's fixed.