Nothing scares you into realizing how fragile our data is until you accidentally delete something important.
In my case it was trying to get some pictures off my phone; the bluetooth connection was misbehaving so I figured I'd just pop out the memory card, only it wasn't working in the card reader, so I put it back in the camera to try the bluetooth again when sccccrrrraaaaapppe! it scratches against the slot and gets jammed. I had to pry the card out and when it did it had two small but visible scratches across it, and for a teeny-tiny MicroSD card even small scratches appear to make a difference. I plugged the card back in, carefully this time, but it wouldn't work. I rebooted the phone in case that might wake it up - nothing! I'd just lost about 30 pictures of the family and half a dozen movie clips including some utterly adorable ones.
So let this be a lesson to all of you - back up your data, and don't fool around with important files if you a) don't have backups and b) aren't paying attention to what you're doing.