This one is easy: Go to the morgue and identify a body. No, seriously, I read stories of people who have to view the remains of a loved one after some horrible accident, and I can think of nothing worse that I could be asked to do. Face a grizzly with a switch? Check. Work at manual labor (I'm a banker) until my fingers bleed to provide food for my family? Check. But this? No way.
I have three kids, and soon will have five, with my wife-to-be's two, and I can't bear the thought of losing any of them, or the love of my life. I've been told it's selfish to not want to outlive them, and I guess it is. But as bad as losing them would be, having something like this be the last memory I have of them terrifies me.