I re-enabled secure boot on an existing system. It then refused to boot the installed Debian 7, or my boot disc for Fedora 20 Live. I'm sure Fedora Live passed secure boot on this system before. However there was also an old Ubuntu install, which was able to get as far as GRUB (if I selected Ubuntu from the UEFI boot menu).
I was surprised because this doesn't match my vague understanding of Secure Boot. How could this happen?
The headline for UEFI secure boot is that linux distros use a key derived from Microsoft. I don't remember manually installing any secure boot keys on this system, i.e. that would let Ubuntu boot but not Debian/Fedora Live. So I wonder what sort of automatic processes might have caused it?