So-called “stupid mistakes” are really failures of character. This dawned on me over 30 years ago when I was tutoring a bright teenager. I was using Birkoff and MacLane to teach beginning group theory to the boy. In exchange his mother was baking me a loaf of bread per week. I was literally teaching group theory for bread. But I digress. One week the boy had not done his homework. He said he was stupid. I replied that no, not doing his homework did not mean he was stupid. It had nothing to do with his intelligence. It had to do with his character. It was about breaking a promise, not being organized, being lazy, lacking of time management skills or an inability to prioritize.
I had a really good bridge partner once whose pre-tournament pep-talk included an exhortation not to make mistakes that we knew about. In other words play up to our capacities. Making mistakes in the realm of “known knowns,” to use a Donald Rumsfeld concept, is a failure of character but is not a reflection on one’s intelligence.
Now when I hear students talk about stupid mistakes, I always point out that the mistake did not mean that they were stupid – maybe careless, inattentive, lazy or just tired but not stupid.