Tinkering is all fun and games, until it's 4 am, your vision is blurry, and thinking straight becomes a non-option, or perhaps you just get overly confident, type something and press enter before considering the consequences of the command you're about to execute... And then all you have is a kernel panic and one thought bouncing in your head: "damn, what did I expect to happen?".
Off the top of my head I remember 2 of those. Both happened a while ago, so I don't remember all the details, unfortunately.
For the warmup, removing PAM. I was trying to convert my artix install to a regular arch without reinstalling everything. Should be kinda simple: change repos, install systemd, uninstall dinit and it's units, profit. Yet after doing just that I was left with some PAM errors... So, I Rdd-ed libpam instead of just using --overwrite. Needless to say, I had to search for live usb yet again.
And the one at least I find quite funny. After about a year of using arch I was considering myself a confident enough user, and it so happened that I wanted to install smth that was packaged for debian. A reasonable person would, perhaps, write a pkgbuild that would unpack the .deb and install it's contents properly along with all the necessary dependencies. But not me, I installed dpkg. The package refused to either work or install complaining that the version of glibc was incorrect... So, I installed glibc from Debian's repos. After a few seconds my poor PC probably spent staring in disbelief at the sheer stupidity of the meatbag behind the keyboard, I was met with a reboot, a kernel panic, and a need to find another PC to flash an archiso to a flash drive ('cause ofc I didn't have one at the time).
Anyways, what are your stories?
Ubuntu GUI/apt fail
Back when I used ubuntu, Unity was stuck with old gnome packages. This meant that the version gnome-terminal packaged with ubuntu (up to at least 18.04) didn't have text reflow on window size changes.
You could add the upstream sources, upgrade the specific text reflow package only, and then disable the sources.
I forgot to disable the sources, or typed dist-upgrade (this happened multiple times...). Broke the whole desktop/lightdm setup with half upgraded packages, and half removed packages (for preparation to install new versions). Way easier to reinstall the os than to disentangle. Unity was a mess then anyway.
Moral: Actually read the package change summaries when doing updates/removes/installs, and [ y/N ] means actually check what the fuck you think you're agreeing to.
BtrFS snapshots for idiots
I've also run automated snapshots on my btrfs partition, then run out of space doing multi-hop system upgrade on fedora (dnf has a plugin that creates a snapshot every time it kicks in.
You can imagine there were many changes happenning per snapshot, and I effectively could have rolled back 4 major fedora versions... Til I ran out of space.
I couldn't get a replacement drive in time, and I had an hour to rebuild my laptop before needing to be on a customer site, so sadly I couldn't preserve my drive for later investigation. My best guess is the high-water-mark was configured incorrectly, and somehow it was able to 'write' data past the extents of the filesystem.
Rollback did work for my home partition, but I had to mount it from another OS to get it to work - so no data loss!
By that time I'd already reinstalled the os to the root partition/subvolume however, so I couldn't determine the exact cause of failure :(
Moral: Snapshots are not backups, and 'working' is not 'tested'