Emma Emmerich in Metal Gear Solid 2.
The game, for me, is superb. My favourite among all Metal Gear titles. The story is awesome, gameplay is complex but fluid, albeit easy bosses (yeah, well), killer soundtrack and not particulary long, but with high-replayability (though Metal Gear Solid 3 has by far better extras).
But, oh man! Protecting Emma Emmerich was almost game-breaking for me. She was just lazy. Her AI was just braindead, and then there's that horrible sniping sequence. And no matter how much you tried, the fucker still dies!
But thankfully, it didn't last long, so you could keep playing (after eating a 35-minutes cutscene that came after)