The phrase only has meaning because it's an innuendo. I think you know more than one meaning for the word "nut". When you use that phrase, the implication is "The problem was caused by human error, and the human in question is mentally ill (a nut).". And maybe you don't think that, but you don't think there's plenty of people who DO make that inference?

It's the same thing as the ID-10-T error. It wouldn't have the sense of "human error due to mental illness" it didn't have a double-meaning, just like ID-10-T has the double meaning of "idiot". This is a pretty basic facet of natural language that middle school children understand.

I've been at jobs where people have used the exact phrase that you used so that they could insult me to my face about my mental health. When I call them out on it, they hide behind the same sort of explanation you're giving now.

Of course, I am not accusing you of doing that, because you weren't. But what you are doing is making the phrase socially acceptable, and thus giving assholes a cover to use those phrases with plausible deniability. It's kind of the same thing (although not as severe) as the people who think it's a good idea to still have Confederate flags flying around.

I don't expect to change your mind on this issue in this conversation, but I hope you'll at least try to understand my point of view.