Two weekends ago I upgraded my Ubuntu desktop from 22.04 to 24.04.3 and was left with an unusable system because I opted to keep my existing copy of the gdm-smartcard-pkcs11-exclusive configuration file because I don't use a smartcard.
But it's a new configuration file and is REQUIRED. By saying I didn't want it updated, the update program didn't create the new one. And since there wasn't an old one, the upgrade failed with "error: alternative path /etc/pam.d/gdm-smartcard-pkcs11-exclusive doesn’t exist" and "The upgrade has aborted. Your system could be in an unusable state." Oh, it certainly was.
It might as well have said, "Enter N if you want your system to become unusable."
The upgrade program should never have asked. If the file is required and it isn't there, it should have just created it. I think it's a bug in the update program.
gdm3, ubuntu-desktop, and ubuntu-desktop-minimal weren't installed. PAM was not set up. No way to log in.
I wrote a blog post about how I recovered from this in case anyone else is bitten by this same issue: https://jerry.hear-me.blog/ubuntu-22-04-to-24-04-upgrade-failure-missing-file/