From Halopedia, the Halo wiki
The Slow-Motion Grenade is a glitch on the Halo 2 multiplayer map, Zanzibar. The effect of the glitch is a slow falling grenade, and has no tactical use whatsoever, until it touches the ground, at which point it will detonate.
Stand next to the non-lowered bridge in Zanzibar. Make your character look up, and throw a Grenade (either Plasma Grenade or Frag Grenade, both work in this situation) towards one of the corners at the top of the bridge. As the grenade hits the bridge, it begins to slide down very, very slowly. However, if you lower the bridge, it will return to normal speed. Once it reaches the ground, the grenade will detonate.