I’m not sure how funny this will be, but here’s how I broke my system twice in a single case. Step by step:
Migrated from Manjaro KDE to EndeavourOS KDE. Kept the previous home directory.
After a few updates, there was a problem with Plasma. Applications were not starting from the panels or the .desktop files (they worked from the terminal. The terminal emulator was in startup and worked that way)
After a few google searches, found out that downgrading glibc would do something, so downgraded… Worked for a while
While using pacman -Syu, I always checked for warnings (foolishly thinking that the downgraded and ignored glibc would cause a pacman warning if it broke dependencies) and there were none. So, the updated OS stopped working due to unmatched glibc. BREAK 1
To fix it, I opened one of my multiple boots (another EndeavourOS) and made a script using pacman -Ql and cp to copy new glibc related files into the broken system (because I was too lazy to learn how to do it the correct way with pacman and chroot didn’t work because glibc is needed by bash).
Turned out the script I made was wrong and I hadn’t checked the intermediate output from pacman -Ql, which was telling cp to copy the whole /etc /usr and other directories. (just if I hadn’t given the -r to cp) BREAK 2
In the end, I just made a new installation, this time with a new home and hand-picked whatever settings I wanted from the previous home, Viva la multi-HDD