I'm installing 10.5.6 on the family MacBook (white) and it was running 10.5.4 previously, yet hanged when I was updating everything (there was a few updates) so I had to force power off. The Mac says it is running 10.5.6, but I wanted to apply the combo update to be safe. However, it fails when installing (it doesn't get to the stage where it reboots, still in OS X at this stage). It says "The installer could not install some files in "/". Also, repairing Disk Permissions failed too. Prior to the update, everything was working fine - I'm usually full of ideas, but when it is your Mac that is playing up I'm very keen to hear what others have to say.