Ultimate sacrifice
Well...there's two ways of looking at that question. The first, and most obvious, leads me to say, No, nobody in any of the games I've played in has ever made that kind of sacrificial gesture.
The second? Well...I've sat through a lot of gaming sessions where players decided to do stuff that very well could have resulted in their character's death, for the sake of the party or even just for one other individual character...and then seen the dice give them a miraculous save. As an example, a Star Wars game I played in for years...our group had an aspiring Jedi, who was actually getting good enough to be something of a threat. She was abducted by an Emperor's Hand, with apparent plans to turn her to the Dark Side and make a Sith out of her...
Well, this particular character had a long history of skirting with the Dark Side...so much so that, at various points in the history of the group, there were characters who assigned themselves to be 'watchdogs' for her...keep her safe--and if she starts going too far to the Dark Side, stun her (or kill her, if the stun didn't work). And this wasn't a secret, either...her player knew all about it and actually thanked them on several occasions for keeping her from losing her character. At the time of this particular abduction, her watchdog was a bounty hunter, who tracked the Emperor's Hand back to her ship, managed to sneak aboard, found the Jedi...and decided to get rid of the Emperor's Hand, knowing quite well that she likely outclassed him in many respects. And when he found her, she moved to attack...
And the bounty hunter fired his grenade launcher at the ground, right between them. Took her out. He walked away with a few bruises. There was no good reason he shouldn't have died, as well. Yes, he had on armor...but more important than that, his dice had a strong sense of karma that night. Rescued the Jedi and continued watchdogging her for another couple of years before the game finally died off from players going separate ways.