So my stepson Walker has a PowerBook G4 that he got from his dad, which stopped booting up. It would get maybe a third of the way through the progress bar labeled something like "Mac OS X" and then it would go to a blue screen (light blue, not BSOD-colored) and hang.
My son Ben discovered that you could boot in single-user mode (holding down apple-S while booting) and repair the disk with "fsck -fy" (perhaps several times), and then rebooting would work. In fact, rebooting turned out to be fine in general, the problem only happened when you powered the thing down.
Well, this was something of an odd problem—the hard drive worked fine while the machine was running but seemed to get corrupted by shutting down. I hoped that reinstalling the system would work, and Apple was kind enough to supply copies of the missing install discs even though the warranty is long-passed. (Though they did send two copies of disc 1 and no copy of disc 2... and a manual that said "PowerBook G4" in the title and on the same page said "PowerBook G5" in the subtitle... hmmm).
So I reinstalled the system and the same thing happened... except that now fsck found an "Invalid Sibling Link" error, which it couldn't recover from (despite looking at this page, which many others found lifesaving), so the thing was broken worse than before! (Yes, I zapped the PRAM, I reset the PMU, I smeared the chicken entrails...)
OK, so there must be some bad blocks on the hard drive, right? A bit of google-assisted head-scratching told me that the mac way to deal with this was to erase the disk, zeroing out the data (under security options) and that this would take all the bad blocks out of circulation. That did nothing. I didn't really believe it, so with Ben's coaching I downloaded an Ubuntu live CD and ran badblocks... which confirmed that, in fact, the hard drive seemed to be absolutely fine.
Some of the web searching I'd done suggested that the Sibling Link error often started up when people switched to a journaling file system, or when there was a problem with the PRAM. So I reformatted the drive on installed as a "Unix format" (ext2?) and since then it has worked like a charm.
My best guess as to what caused the problem? I'm guessing that the PRAM battery is dead, which can cause "unpredictable symptoms." Looking around the web I haven't seen anything about corrupted hard drives, but it's hard to understand what else would cause a disk problem only on shutdown, with a hard drive that passes all the other diagnostics with flying colors.