I recently purchased Windows 8 and installed it on my machine. I thought I made a backup of some old data that I've archived, but it turns out that something got seriously fucked up and I lost maybe 18-19 gigs of personal info - music, documents, music projects, videos (I manage an indie dubstep producer's Youtube page, and lost all the videos AND custom artwork I did for each song), graphic design projects, lotsa shit that I can't get back. I even lost an avatar that I made a long time ago that I thought I might use here. Found the original source .pdn file :D. I can't fucking believe that I lost all of that shit.

Edit: Turns out I had a couple copies of some files, so it's not a 100% loss.