It must have been many years ago, but I have recollection that someone once handed me a newly-manufactured circuit card and asked me to help figure why it didn't work. I glanced at it for about one second, pointed to a resistor and said 'that resistor is the wrong value.' I was right. They were amazed.
The resistors on that card were normally all marked with only the darker colours like brown, black and red. The out-of-place yellow mutiplier band stood out to my eyes.