We had someone show up to a scrim with some homemade grenades once. They didn't ask if they could use them, they just went ahead and did it anyways. Luckily they didn't actually land near anyone because they were of such power that they would have taken someone's fingers off. They had basically taken apart fireworks and made them into a makeshift frag. It was in such poor shape that gunpowder was leaking out of kinks in the fuse which could have short-fused the thing while it was still in someone's hand.
Seriously, guys. I've been in the explosives industry for a long time and even I don't make this kind of stuff. It's not safe or smart.