Ok, I watched the demo, and I'm not sure how that would happen. I double checked the code and, on contact with anything that takes damage the grenade will:
a) Act like a direct paintball hit, so it's impossible for it to randomly miss.
b) Immediately explode.
What happened in the demo is double-strange, since the grenade bounces on you and not only doesn't eliminate you, but it also doesn't explode. Only explanations I can think of are.
1) You were playing on a server with a really old version (last grenade change was in game version 1.924, some time in 2009).
2) The server somehow got into a corrupted state -- like the grenade entity somehow was not considered a grenade anymore, or your player entity wasn't considered able to take damage.
Would be really interested if there was a way to consistently reproduce this.