20+ years ago I was primary support for the state's ambulance dispatch system - 000/999/911 calls come in and are sent to the ambulance dispatcher to send a resource. For some reason we were doing an upgrade in the early morning (less emergency calls in case something goes wrong) and as a precaution my off-sider was on-site as well. It was a home-grown Pick system running on Pr1me minis with a 3GL green screen front end. The upgrade went fine, but while I was looking around the system I saw a file that I thought wasn't needed so I deleted it. Immediate chaos - everything stopped. Mad scramble for paper and pencil so emergency calls could be recorded and ambulances dispatched. Fortunately we had a live backup system so after a few minutes of panic, we copied the missing file from the backup system and restarted and everything's back to normal.
Always got by off-sider to look over my shoulder after that and he didn't like playing with the system.