My life plus the life of a random stranger, for example. If I was doomed to die in a certain fashion but had the chance to save another life (even in a way nobody would ever know about), well, that's a no-brainer for me.

EDIT: Ah, now I see the context. How about the following hypothetical:

I am on a spaceship returning to Earth when all my shipmates die. I realize that I am a carrier for a horrific disease; I will never get sick from it, but I can transmit it to others, of whom 99% will die. Let's furthermore imagine that the people on the ground don't know about this yet, that I have good odds of surviving if I just land somewhere and make a run for it, and that no effective quarantine exists short of self-destructing the ship before I land.

If it's therefore reduced to "I die" versus "I survive, but cause a mass extinction event", I think I self-destruct the capsule. Perhaps not without some angst, but it's still an obvious choice to me.

N.B: Given the many cases in history where intelligent people in dire circumstances have accepted death (or high odds of it) on behalf of something they see as more important, I think the case that revealed preferences sometimes value things above one's own life is pretty strong.