This morning, as I ran the x86_64-current update, the computer suddenly rebooted just after the appearance of the KDE splash screen. This is not the first time this has happened, but it is very infrequent. Everything started normally after this unexpected reboot.

It has never seemed to have a detrimental effect, so I have no idea if this is a problem or not.

I'm just curious if anyone else has seen this behavior.

Also, my default run level is 4. Yep - I'm a lazy Slacker.

While I have no specific explanation for the unexpected reboot, I consider it generally a bad idea to update Slackware while X11 is running.
This is because many of the currently running processes will have the old version libraries they initially loaded with removed while everything is still running. This can lead to crashes and unexpected behavior. Data corruption is certainly possible.

Consider switching to init level 3 and terminating any non-essential processes before updating in the future.

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I like to also switch to init 1 and back to init 3 or 4 after the updates are applied. Guarantees that all processes are restarted after an update.

X was not running - I was in init 3. One cannot install or uninstall an Nvidia driver in init 4. Also, there was a kernel upgrade that would cause one to reboot after adjusting lilo.conf, making an initrd and running lilo.The first reboot was to init 3 after editing lilo.conf and making an initrd. From there I ran mkinitrd_command_generaror.sh and reinstalled the Nvidia driver. Now I rebooted for the second and last time to init 4. That was when the screen went blank after the first icon appeared on the KDE splash screen, and the unsolicited reboot happened.

Thanks anyway

I have seen odd behaviour on a Slackware box when I had a bad stick of RAM. Perhaps you can run memtest and check your RAM. A faulty power supply can also give odd behaviour. I have 5 Slackware64-current stations and an OpenBSD laptop. I didn't observe the rebooting behaviour after the latest update.

Ok, thanks for the clarification. Your original post gave me the wrong impression.

I have had the screen go blank like that when loading KDE the first time after upgrading nVidia drivers and rebooting.
In my case it was KDE's display compositor crashing. When this happens KDE will typically disable compositing for the next time you log in.
This may not be the root of your problem, but certainly something to look into.