Somehow, there is a curious fact that sometimes with Get Info or Disk Utility, you see that you are almost out of disk, but when you go to the Finder, it looks likes things are fine. What is going on? Apple has a new feature with Time Machine Backups where where it creates an invisible file in /Volumes/MobileBackups. This stores files that need to be backed up by Time Capsule. When you need more space, this is automatically removed. You can see this by going About This Mac and clicking on More Info and clicking on Disk. On my 500GB drive, nearly 40GB is devoted to this. On Ales' 120GB drive a third of the disk is used for this.
To turn this off, you have to utter this command:
sudo tmutil disablelocal