IBM Customer Engineer
Back in the early 80's I was working for a major bank. We had 2 identical IBM System 370/145s. Our production system ran the on-line system for every branch in the state (the second system was always powered up and usually running dev workloads). Fortunately we swapped systems each day to ensure that both were fully operational and capable of running the production system.
We had a IBM customer engineer (CE) on site most of the time. One day the usual engineer bought in a new trainee to introduce to the staff. When the normal CE went on holidays the newbie was to take over for a couple of weeks.
On the first day the new CE was running solo he was walking past the production system, looked at the "Big Red Switch" (actually white with red lettering) that was at the top right hand corner of the main console labelled "Emergency Pull", commented "what's that doing in?" and, before anyone could stop him, he reached up and pulled the switch. The role of the "Emergency Pull" was to cut all power to the system immediately. It did this by tripping every circuit breaker in the system. The room went into "panic mode". We knew we had 15 minutes before the phones would start ringing off the hook (the branches had instructions to wait 15 minutes before calling the computer centre in case of an outage). All jobs on the back up system were cancelled and job queues flushed. The "on-line" system was bought up on the back up system as fast as we could and service restored just before the 15 minute deadline.
The CE was marched out the door and told to never return. Our IBM rep was called to reinforce the order. Another CE who had experience with our site was called in and spent the next 2 days cursing his colleague as he worked to get the system back up.