Do not forget: "After an update, KDE was just a blank screen with some red X's after login. I thought something went horribly wrong with the update. df -h revealed the root was full, pacman -Sc freed up plenty of space and all was well after reboot. Figured I'd pass this along to others who are as negligent as I am in this regard and perhaps spare a freak-out or two." - http://www.reddit.com/r/archlinux/comments/14s999/just_a_friendly_reminder_to_clean_up_your_pacman/
Happened to me this last week. In another moment (last year) I couldn't even update because of that, I would get several errors.
It is always smart to remember (if you are an ArchLinux user):
1. Update the system everyday - just be careful with kernel updates (took me some time to learn that...).
2. Clean up pacman cache with a certain frequency. Again be careful: "Only do this if you are sure that the installed packages are stable and that a downgrade will not be necessary" - Arch wiki.
3. Make sure to read the Pacman page in the wiki: https://wiki.archlinux.org/index.php/Pacman.