Here is an example:
A bat and ball cost $1.10. The bat costs $1 more than the ball. How much does the ball cost?
The obvious answer that immediately comes to mind is $0.10. But if you check, you will find that's wrong. If the bat costs $1 more than the ball, and the ball costs $0.10, that means the bat costs $1.10, so the bat and ball together costs $1.20. The correct answer is $0.05 (with the bat costing $1.05).
Our intuition can't be trusted, but we are all inherently lazy when it comes to mental exertion, which can lead us into all sorts of mistakes. The book deals with cognitive bias, the heuristics we use to arrive at wrong answers, and over-confidence in our intuition. It also has some interesting insights into how pain and pleasure are experienced vs how they are remembered - our experiencing self and our remembering self tend to disagree on what we've been through. Highly recommended.