Besides runaway log files that aren't being properly rotated, human error can cause it too. I managed to completely eat up the disk space of one of our staging servers a few weeks ago trying to tar up a directory so I could work on it locally. Didn't realize the directory was 31GB and we only had 25GB of space. By the time the notification for 80% usage was triggered (no more than 2 minutes after we hit 80%), the entire disk was full. Luckily it was just a staging server and no real harm was done, but such a mistake could have just as easily been made on a production server. In this case, the obvious solution is to just delete the file you were creating but if you're running a more complicated process that is generating logs and many files, it may not be so easy and this 8GB empty file might be useful after you cancel the process.