Back in the 80's I worked for a field service organisation, fixing and maintaining PDP11 and VAX systems, but also CDC-9766 removable disk systems. Big 14" removable disk packs like you see them in old scifi movies. One of my customers had a string of 10 or so attached to a five-node Tandem Non-stop system.
Each week they brought two out of ten off-line for me to work on. I cleaned the heads, then used a servo disk pack to realign those heads.
To do this, I needed to remove the control cable from the string, and plug in an excersizer. One day I forgot to pull the control cable. So instead of moving the heads of my offline drive to a specific track, I moved the heads of *ALL* disks in the string! Without the O/S knowing about it
Believe me, that will bring a Tandem Non-stop to a grinding halt. That was my last time on the floor for that customer, but I didn't lose my job. Cost? I don't know. Perhaps a weekend of data recovery for the operators?