At my old job, me and a colleague were tasked with designing a new backup system. It had integrations for database systems, remote file storage and other goodies.
Once we were done, we ran our tests, and sure enough. The files and folder from A were in fact present at B and properly encrypted. So we deployed it.
The next day, after the backup routine had run over night, I got to work and noone was able to log in. They were all puzzled.
I accessed a root account to find the issue. Apparantly, we had made a mistake!
All files on A were present at B... But they were no longer present at A.
We had issued 'move' instead of 'copy' on all the backups. So all of peoples files and even the shared drives have had everything moved to remote storage :D
We spent 4 hours getting everything back in place, starting with the files of the people who were in the office that day.
Boss took it pretty well at least, but not my proudest moment.

*Stay tuned for the story of how I accidentally leaked our Amazon Web Services API key on stack overflow*

/facepalm