Playing on hardcore, was going great and had lots of good stuff.
Found a hole in a desert that had lava just under it. Dug out one block which collapsed all the sand around it, dropping me into the lava, killing me.
The weirdest thing? I could have sworn I heard laughter when I died.
The noobiest thing I've done was when I was a week into MC. I went searching for diamonds and STUPIDLY forgot to drop off all my items in my chest upstairs. I had all of my diamonds, some of my diamond tools, several stacks of iron and gold, a clock, a compass, a map, my last few stacks of gravel, 20 obsidian, and a water bucket. I found some diamonds and decided to dig around them to make sure there wasn't any lava. There was lava. I had a water bucket in my hotbar, but I had the BRILLIANT idea to drop some gravel in. I sneaked over the gap and placed some gravel. Because I was sneaking and placed a block under me, the game thought I had walked over a new block. Since that block was gravel, I fell into the lava. Sadly the lava was so deep that there was nothing to stand on to save myself. I died, along with all of my items.