While upgrading the kernel, my raspberry pi reset and failed to
come back up to a usable state. Serial console showed the last thing
output was the kernel saying it was running init. Investigation of the
SD card found this in config.txt (abbreviated to relevant lines):

kernel=vmlinuz-6.3.0-1-armmp.dpkg-new
initramfs initrd.img-6.1.0-9-armmp

Note the kernel and initrd version mismatch.
(The .dpkg-new being used in there also seems a bit problimatic..)

I had to edit it back to using vmlinuz-6.1.0-9-armmp to fix the problem.

It seems to me that since rpis are known to be rather picky about
power[1], it's especially important to make such a critical thing as a
kernel upgrade not fail if the power goes out.