I also have a couple friends that require serious thinking (or being on my toes). I think it’s because they have some model of how something works, and I say something, showing my lack of this model.
Additionally, programming causes this as well (in response to compilation errors, getting nonsense outputs, or runs too long).
From an apparent author on reddit:
The comment was responding to a claim that Terence Tao said he could only solve a small percentage of questions, but Terence was only sent the T3 questions.