The recent GRUB2 patches that were released to mitigate critical BootHole vulnerabilities also caused boot failure issues for some users. Luckily, fixes for these regressions have started appearing for some distros, including Debian and Ubuntu.
Last week, I was reporting on the BootHole vulnerability (and some other seven flaws) found in the GRUB2 bootloader, which is used by almost all GNU/Linux distributions out there. The issues opened up systems using Secure Boot to attacks, allowing local attackers to bypass UEFI Secure Boot restrictions and execute arbitrary code.
Due to a highly coordinated effort between the security researchers who discovered the vulnerability and Linux OS maintainers, most GNU/Linux distributions were able to provide patches for their users. However, for some, these patches broke the Secure Boot implementation and left people with unbootable systems.