In college I worked for a small time consulting firm, writing dBase code. One of my projects was to write a backup module for an existing program. I made the system smart enough to know which files it needed to backup, and where they were, and how to split files across disks. After completing the fully customizable and ultra flexible routine, I installed it at the client site.
I watched the secretary do a complete backup without my interfering with or leading the process. It prompted her for which drive she wanted to backup her information onto. She put a disk into the A drive, responded "A", and the process began. The system erased everything on the backup drive and filled up the first disk. After changing disks, it finished the backup.
The next day I got a phone call. This is what apparently happened:
The second time through the backup process, the secretary didn't read the initial backup "what drive to backup ONTO" question. She put a disk into the A drive and responded that she wanted to "backup drive C". It proceeded to erase everything on the disk she was backing up onto (drive C is what she had entered), then it discovered there was nothing left to backup from drive C, so it was done. At this point their original files were gone, but they had the pair of backup disks from the previous day.
Since system finished it's backup in a few seconds, the secretary assumed something was wrong and started the backup a second time. She read the initial question this time and realized her mistake. This go round she properly told it that she wanted to backup "onto drive A". She made sure that the first disk of her backup pair was still in drive A, and continued. The first thing the system did was erase everything on that first backup disk. Then it discovered that there was nothing on drive C to backup, and finished.
They had lost a day and a half of data entry, had none of their database or program files, and half of their backup set was now empty. This was about the time I got that phone call.