--- spoiler inside ---
in the very end of half life, after you killed the nihilanth, you're 'teleported' (what's the word?) in several places with the g-man who explains you some useless things (can't remember what actually), and then you're in one of the shuttle things (subway?) flying in space, the g-man asks you to make a choice, jump in a green teleporter or stay in the shuttle. if you stay in, you're teleported right in front of a lot of aliens, and of course you don't have any weapons, fade to black. if you jump, fade to black, the g-man tells you that you will meet soon, you're gonna work for him o//
don't know if it's really clear lol.. it's been a long time i finished this game