As a magician I am essentially an eccentric student of human psychology. The cognitive biases that we all share and lead our thinking astray are essentially how most magic tricks work.
Dr. Steven Novella maintains a wonderful blog called Neurologica where he writes extensively about these sorts of things. He recently wrote an article describing a new bias, that I had never heard of, but instantly recognized: The Mechanism of Substitution.
Before being a professional magician, I spent my last year at school working primarily as a math tutor. Essentially it was my job to sit and watch young people try and solve problems (and for the most part problems they were either not yet, or just barely, qualified to solve.) It happened all the time. They could interpret the question and without realizing it they would edit out parts that made the problem more difficult and start forging ahead on the simpler problem, oblivious to the difficulty they had sidestepped. (It actually feels like a sense of accomplishment until you check the answer in the back of the book and couldn't figure out why you were wrong.)
It is, not coincidentally I think, how most magic tricks work. We generally set up false expectations so the the audience is trying to solve the wrong problem. As a great Canadian magician, Dai Vernon, once put it, "You have to lead their thinking astray."
The full article is certainly worth reading.