The one I remember best was a guy who'd had a discussion with one of his buddies on the subject just a few days earlier and thought that using a helmet to confine the blast might allow the person who jumped on the grenade to survive. A few days later, while out on patrol, his squad encountered a group of terrorists and got into a gunfight with them. While their concentration was focused on these terrorists, another snuck up behind them and chucked a hand grenade into their midst. This Marine (I believe he was a lance corporal) spotted the grenade, ripped his helmet off, and dove on the grenade. The grenade partially muzzled the grenade's blast and this heroic Marine might have survived, if a single fragment hadn't struck him at the base of his throat and penetrated into his brain. He was medevaced to a hospital where he died almost two weeks later.
I know there were several other Marines who made this ultimate sacrifice, but this is the one that has stuck with me over the past few years.