Once, a long time ago, I spent the best part of a night writing a report for college, on an Amstrad PPC640 ( http://en.wikipedia.org/wiki/PPC_512 ). Once I was finished, I saved the document -- "Save" took around two minutes (which is why I rarely saved). I had an external monitor that was sitting next to the PC; while the saving operation was under way, I decided I should move the monitor. The power switch was on to…
That was back in the summer of 1978; today I have an LTO-4 tape drive driven by Bacula and backup the most critical stuff to rsync.net, the latter of which saved my email archive when the Joplin tornado roared next to my apartment complex and mostly took out a system I had next to my balcony sliding glass doors and the disks in another room with my BackupPC backups.
As long as we're talking about screwups, my ... favorite was typing kill % 1, not kill %1, as root, on the main system the EECS department was transitioning to (that kills the initializer "init", from which all child processes are forked). Fortunately it wasn't under really serious heavy use yet, but it was embarrassing.