In a similar vein to @Awlex's story, I lost 800GB of anime in a disk head crash. To this day, I have no idea how it happened.
We were getting our house painted and so my anime hard drive was left untouched for about a month, and then when I tried to connect it I found the disk head had crashed.

And then, a second time when I had managed to amass another 400GB of anime and something went wrong while I was trying to delete some other folders through Ubuntu's GUI (yeah, I know) and my anime folder was accidentally shift-deleted. T-T

Now my collection is back up at 300GB and I only use command-line to delete multiple folders now.