My worst would have been catastrophic if I had waited one minute to make my mistake.
I was commissioning a new control system at a power plant's water treatment facility. I was fairly new to the industry and had mostly looked over the guy who did the bulk of the work's shoulder as on the job training.
This particular day the guy was out sick and we had to finalize a couple of things before we ran through the final tests.
There was an instruction to open a valve to fill a tank and it had the wrong variable linked to it. The problem was to maintain the naming standards I had to do a download to the processor to make the change. When I had been doing work in the office this was not a big deal, download the program to the processor, it stops running for a moment while it loads the new logic into memory and starts back up.
Not thinking through the implications of the processor shutting down while the process was up and running I made the code changes, hit download and about 30 seconds later an operator came running over looking like he had seen a ghost and he was pissed.
While I was making my code changes the operator was hooking up a hose to drain a rail car of some chemicals. The way the valves were configured before I made my changes was correct and would have had no consequence it I didn't touch anything. The way the valves were configured when the processor restarted would have routed the rail car's contents to the wrong tank resulting in a reaction which would have created a huge plume of highly toxic gas. The way the wind was blowing this plume would have blown directly to the largest town in the area and could have killed a ton of people.
The operator heard the valves in question changing position before he opened the valve on his hose to empty the rail car and figured something was up. When he saw the whole process had shut down he got really angry because I had ignored the protocol in place to avoid such a disaster.
I got chewed out and kicked off the site. My boss attributed my mistake to inexperience and I had to give a safety presentation on what I did wrong.
Lessons learned:
Be sure you are aware of any implications your actions have. If you are unsure or guessing about something stop what you are doing and go ask someone first.
Don't give people mission critical work on their first project and have them work unsupervised. Training is important.
Always be aware of safety requirements, especially when you are working with machinery, automated processes, chemicals or anything else that can hurt, maim or kill you.