Had a large haul while mining, turned a corner and ran into a creeper. It didn't kill me, but I freaked out and ran the other direction... right into a lava pit. Everything I was carrying was instantly vaporized as result.
Week and a half, 53mb save file, 7000 rail track long tunnel, was looking to replace my floor with something else so I used an inv editor to try out different blocks, not knowing Adminium was unbreakable (even though I've hit the bottom while mining time and time again) I placed a block in the middle of my house and was annoyed because I couldn't get rid of it so I used another inv editor and decided that changing all the Adminium to Rock was a good idea so I could simply break the block, so I did it but it messed my save up and caused the game to freeze if I moved once the save was loaded (everything was also really dark)
Making the best house I ever built, going out to explore, falling in an un-expected mineshaft, getting out, losing my sense in direction, and spending over an hour wander across the world trying to find my house.
One time I quit the game while I was up on my tower (no guardrail) looking out at the night sky. The next day I loaded the save up and due to a virus update running at the time, lagged off the edge of the tower into the moat. I was at the respawn screen before I realized what had happened... so I ran back to get whatever was left from the edge of the moat, jumped up to get the stuff and a shot me, knocking me in again. Got the respawn screen over my inventory screen which was interesting. So I ran back and grabbed my spare bow to take care of that darn :Skeleton:, leeeeaned out over the edge to try to see him... guess what happened (this was before crouch)...