Some of you may have experienced this bug: If you start the BeOS Demo CD after having already installed the full version of BeOS, the bootloader may be corrupted (this should be fixed in future versions of the Demo CD)
To remedy this, boot the full version with the Demo CD (press and hold the Spacebar while booting and choose the volume containing the full version from the menu). Open a Terminal and type:

 makebootable /boot 

The BeOS should now start again with its usual bootloader.

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Kristof Polleunis chimes in with the following:
In rare cases, the BeOS can spontaneously bork the bootloader on reboot, causing you to have to run makebootable quite a bit. To subvert this, and make sure nothing corrupts the bootloader on shut down, you can add the command makebootable /boot to your UserShutdownFinishScript.