If you start up from the Mac OS 8.6 CD and then put the iBook or iMac (Slot Loading) to sleep (or let it go to sleep by itself), it will not come out of sleep properly. Instead, the hard drive spins up but the screen remains black.
On an iBook you can press Command-Control-Power or press the reset button to recover. On the iMac (Slot Loading), press the reset button on the side of the computer, or press the power button on the computer itself for at least 5 seconds to recover.
This behavior does not occur when starting up from the Mac OS 9 CD.