If you think your Mac OS X is as solid as a building brick, you are wrong. Something shit can happen that causes OS X fail to load.
got this crash experience recently with my OS X. As technical spec, I
have MacBook Core 2 Duo 2.0Ghz, DDR 1.5 GB, HDD 80 GB, with OS X
Leopard 10.5.2 before crash. The initial cause of problem was my
Macbook battery not detected anymore. Then in some bright days the power
adapter had got unplugged accidentally, so the Mac OS got to sudden
shutdown. This happened several times, so in one day I tried to do
Verify Disk via Disk Utility. It told me some errors. Then when the
10.5.3 update was released by Apple, I downloaded it and tried to
install the update in high hope that it can fix my problem. Totally
wrong! The update only went half of the way just to say something error
to update my root folder. Then it freezed, Finder couldn't be used at
all. Then I pressed the magic power button for a while to force it to
shutdown. And here was the situation: OS X would not load anymore. It
booted, shown Apple logo, dark screen, and restarted, kept restarting.