An angry customer was breathing down my neck, making idiotic suggestions. I was rattled.
He wanted me to try swapping out some component. I said no, I was 100% sure that was not the problem. I was laser focused on fixing this problem.
Turned out I was wrong. Customer complained to my management and I got a sound bollocking and took a couple of steps backwards in my career.
I learned that day to always remember that, no matter how unlikely, you might be wrong.
As a sidenote, this is one of the 25 points of advice in John Perry Barlows "adult principles" which I discovered late in life. It is the only written work I have on my wall, with items 1, 3 and 10 marked with highlight pen by my wife as areas for personal development for me.