What's the most important thing you've lost due to lack of backup?

Didn’t log in to it for some years and when I did it was all gone.

On one hand I’m kinda glad because I realized all this digital shit is just temporary.

Our human memories are forever.

I carried the drives around in hopes of recovering the codebase for a virtualized+distributed SSI OS (Kerrighed-module-based), which had been in the works for about a year at that point.

Due to changes between 2.4/2.6 and 3.x kernel; the rise of user-level distributed computing in C/RIU, kubernetes+docker, I never really recovered the work: there were partial backups of some of the features scattered across three contributors' systems, but no coherent backup of the unifying components; and well "life" with one of the key contributors becoming 'Justice Impacted' stalled any real progress.

I use this as a personal motivation for RAID!=Backup.

I worked on it all day, then tried running it - whereupon my choice of default output filename (main.cpp) caused it to overwrite its own source code. There were no backup copies.

I told no-one, stayed late to re-write it, and demo-ed it the next day. I like to think I learned some kind of lesson, but I'm not entirely sure what.

Honestly, after 20 some years in technology, I don't think it's possible to back up everything unless you are willing to pay and constantly work at it.