This Missed Connections post on Boston's craigslist drew quite a bit of attention:

I met you in the rain on the last day of 1972, the same day I resolved to kill myself.
One week prior, at the behest of Richard Nixon and Henry Kissinger, I'd flown four B-52 sorties over Hanoi. I dropped forty-eight bombs. How many homes I destroyed, how many lives I ended, I'll never know. But in the eyes of my superiors, I had served my country honorably, and I was thusly discharged with such distinction.
And so on the morning of that New Year's Eve, I found myself in a barren studio apartment on Beacon and Hereford with a fifth of Tennessee rye and the pang of shame permeating the recesses of my soul. When the bottle was empty, I made for the door and vowed, upon returning, that I would retrieve the Smith & Wesson Model 15 from the closet and give myself the discharge I deserved...

Many commenters on metafilter and reddit reported being choked up by the end of the story, in which an old man reflects on a chance meeting that both saved his life and left him forever aching for a lost possibility. Others immediately set about evaluating the truthfulness of the tale. 

I read the story with a skeptical eye, as one must approach anything on the web. The way the author described that night over four decades ago didn't quite jive with my sense of what we remember and how we choose to describe our memories, but I couldn't put my finger on anything specific that called the story into question. Later, the story was fairly effectively debunked. [The link to the full text of the debunking is jacked up right now, and it veers off into a really bizarre screed about Viet Nam, so I'm just leaving it alone. Sorry. You can google it if you don't want to take my word for it.]

What really intrigued me was this comment on metafilter:

The interesting thing to me is that, if this is a true story it is a poignant one, and beautifully written. If it is fiction, it is cloying, and poorly written.

That observation strikes right to the heart of what fiction is, and can never quite be, no matter how hard we try.