Guild Wars a few years back. I was about 20k experience from the Legendary Survivor title- where you need to attain 1337500 experience without dying, no small feat, and this was before they had the reset, so if you died once, that was it. Anyway, I was 20k experience from this title, and I know that I'm going to be able to get it on this next run. I run up to the first mob, and my internet cuts out. When I finally log back on, I find out that my character had died to the mobs, as a result, all that hard work was nullified (I've since got it, Anet made it so it was X exp since last death, not X exp before your first death). Definitely my biggest FUUUUUUUUUUUUUUUUUUUUUUU moment.