One morning, I cracked an egg into a bowl and threw away the shell. I cracked another egg into the bowl and threw away the shell. I cracked the last egg into the trash and threw the shell in the bowl. I realized something was wrong when I was about to whisk together two eggs and a shell.
There were no more eggs in the fridge. I wasn’t going to scoop a yolk and its whites out of the trash, so I was short on protein that morning.
There was no reason to drop an egg into the trash.
I crack three eggs every morning. 21 eggs a week. I stand in the same place, use the same bowls, and keep my eggs in the same shelf on the fridge. I’m well practiced. The circumstances are consistent.
Even though it made no sense and even though I was set up to succeed, I made an unpredictable mistake. How could I have guessed I’d throw away part of my breakfast for no reason?
Human operators will make unpredictable mistakes some percent of the time, even on the simplest tasks. Training and other tools can reduce that percentage, but not to zero.
All operations have to plan for unpredictable mistakes. That means there has to be a way to restart the task. There have to be more eggs in the fridge. You can’t plan to scoop an egg out of the trash.
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