OK, the following is quite odd, but very true, as it happened to my machine just yesterday. More than likely, it was just a fluke, but since the issue persisted across reboot attempts, I thought I would share it with the macosxhints readers, just in case anyone else runs into it.
Yesterday, I was trying to install an ATI ROM upgrade for my video card. To do so in 10.4, you have to start up in Safe Mode, which is quite simple -- just hold down Shift at startup. However, when I tried this on my first-gen Dual 2.0GHz G5, I had some very unexpected results.
The first time I tried to boot into Safe Mode, I could hear the hard drives going into their "first aid" check, which is normal. Lots of drive activity, and the spinning gear on the screen. But then, the fans began to ramp up. And ramp up. And up. And up. Soon there was a discrenible breeze in the room as the G5 sucked in every spare molecule of air in the house. If things continued at this rate, my cat was in danger of becoming a hood ornament on the G5's grill.
Then suddenly, everything got very quiet. Too quiet. The G5 had shut itself down. From what I understand, this happens when temperatures get out of range. I suspected some sort of disk issue had caused the machine to get stuck during the first aid portion of the safe boot. In turn, the fans ramped up to try to keep the disks cool. Not being successful, the G5 bailed.
I rebooted into normal mode, which worked fine, and checked the disks. To my surprise, everything was fine with Disk Utility's repair tool. I also repaired permissions (no issues found) just to be sure. Then I booted into Safe Mode again. Same result; the fans ramped way way up, and the machine shut itself off.
I booted normally again, which worked fine again, and was just about to call Apple and open a trouble ticket. Then I noticed I had a CD in the Superdrive (Nascar Racing, in this case). On a lark, I ejected the CD and then again booted into Safe Mode.
This time, the fan's ramped up, but not to max velocity as they had before. A couple minutes later, the fans dropped back to normal speed, and I saw the Safe Mode login screen. Very strange, and as noted, potentially a fluke. I'm not planning on testing it again, though, to find out! In the future, I'll remove any mounted CD prior to trying a Safe Mode boot.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060308073725467