Ubuntu 12.10 recognizes me only 3GB of free space

My 2 cents

1. Try to use df -hT to see free space, also use df -ih to make sure you have enough inodes on the file system, sometimes file systems can run out of inodes before running out of free space.

2. use smartctl -a /dev/sdX to check SMART info (gsmartcontrol for GUI), make sure the HDD is in healthy condition

3. if the issue happens again, try using find command to find super large files (e.g. 1GB+): find / -type f -size +1G -exec ls -lhS {} \;

4. Remove it interactively (BE CAREFUL): find / -type f -size +1G -ok rm -i {} \;

5. It is possible that some nasty apps were doing something in the background and filled the disk.

BTW: looks like your / and /home are on the same partition/LV. What file system is that? Ext4? I don't think this is a bug unless you can consistently reproduce the issue.