The big things that make you go "oh f*(&, no"

Question

Share those moments when something just went horribly wrong when you were least expecting it (computer-related, preferably not a real-life tragedy).
Before I left the house this morning, I sync'd the laptop with the desktop because I was taking it somewhere. As an afterthought I did an upgrade on the desktop as I hadn't turned it on in almost a week. Halfway through the upgrade the console got spammed with error messages and everything stopped working (and I mean everything... ls, su, etc.), even in a separate tty. As far as I can tell, there was a disk error on /var and it borked the system upgrade completely. After muttering a bit, I had to power off manually and leave it for the evening, during which I spent about two hours backing up data, cloning partitions, checking disks and upgrading packages from a host backup system.
I still have no idea what happened (the logs are empty), but the disk seems fine. The partition in question was a ReiserFS LVM volume that has passed all subsequent checks and from which I was able to create an ext4 clone (there's a bad joke in here somewhere about Reiser murdering my system).
There was a great facepalm moment at the end of it all too... after copying disks, changing fstab and re-installing (via the backup system) all packages that had been updated in the last two sessions, I forgot to update the bootloader to point to the new disk. It took me 20 minutes, at least 2 reboots, and sleep-deprivation-driven questioning of my own sanity to realize it.
All I can conclude from this is that Allan is a sneaky b*$^@&).

Answer 1

Not nearly as bad as Awebb's or ewaller's, but it ate up my entire weekend a couple weeks back:  Since a laptop is all I have, and I don't want to keep the battery charged up at 100% all the time, I'm always looking to squeeze out battery life.  So I decided to move my /home partition from the internal HDD to the mSATA SSD the OS is installed on, and create a new partition on the HDD for all the media I don't use when on battery, so the HDD can spin down and stay spun down.  Before proceeding I made back-ups of personal media, and backed up the / partition using Acronis True Image just to be safe. 
While tweaking the partitions, there was some slip-up in the Gparted live environment that left the / filesystem in a weird state: Even though the resize from 4.5 Gb to 8 Gb was successful, it spit out some error message and the reading showed roughly the same proportion of the disk was filled; i.e. before and after the resize, both Gparted and df showed ~85% of the disk as full.  "No big deal,"  I thought.  "I've seen this once before; I'll just wipe the partition, keep it at 8 Gb and just restore the image in Acronis." So that's what I did.
Trouble was, despite the fact that the image had been successfully created, and Acronis kept claiming it had been successfully restored, the target partition was nonetheless empty.  I tried deploying the image to three different partitions on three different disks several dozen times, always with the same result.  Seriously, I must have spent four hours doing the same thing over and over, because I could not believe that two different pieces of software--both of which I'd used many times in the past, both of which were highly trusted and one of which I'd actually coughed up $45 USD for--would crap out on me in the span of an hour. 
I was forced to reinstall and then restore ~600 Gb from my back-up, but it all worked out for the best: I took the opportunity to further slim down my system, I got UEFI boot working, and I decided to take the opportunity to use BTRFS full-time, 'cuz of course when you drop a chunk of money on red twice at the filesystem wheel and it comes up black each time, the best choice is to double down and spin again.