Objective
Swap two disk UUIDs inside configuration.nix.
The intended change was small and straightforward.
Expected Work
Approximately 30 seconds.
It appeared to be a harmless one-line configuration change followed by a normal rebuild and restart.
Unexpected Side Effect
During the edit, the user nebu was accidentally removed from the configuration or was not correctly preserved in the final version.
After rebuilding, the active system configuration no longer contained the expected user account.
Symptoms
The rebuild completed.
NixOS restarted.
The system booted perfectly.
The graphical login appeared.
The administrator...
...did not.
The mistake only became apparent after the restart, when the expected user account was no longer available at the login screen.
Lesson Learned
Small declarative changes deserve the same attention as large ones.
Always review the final configuration before rebuilding.
Declarative systems are precise.
Humans are not.
Recovery
Fortunately, the original configuration had already been committed to my personal Git repository.
Instead of recreating the configuration from memory, I restored the original version from that repository and rebuilt the system.
What could have become a lengthy recovery became a straightforward restore.
Evidence
The documented screenshot records the visible result after the system itself had booted successfully: the graphical login was present, but the expected user account was missing.
