Here's my worst story:
I was playing with this group of mods called Cosmic mods(I highly recommend it to any and all who are a bit bored with vanilla Minecraft, but don't want anything extremely different). The main mod adds 5 new ores - copper, scrap metal, both of which are common, Mythite, which is really rare and can only be found in high mountains, Unobtanium, which is even rarer and can only be found at diamond depth, and Infinium, which is insanely rare and can only be found on the bottom 4 layers or so. Anyways, after getting my Infinium pick(which digs through just about everything instantly and has infinite durability) I kept on branch mining to find more Infinium for more tools, but at one point when I was making a tunnel through a lake of lava that was in my way I slipped and fell into the lava... And it took me around 7 hours to find the 3 pieces of ore to make the pick. Thankfully, I had a relatively recent backup.
The moral of the story is: Kids, always backup your Minecraft saves!