We've all been guilty of giving a shallow review: "Looks ok."
Given typical defect densities, any non-trivial design or code is going to contain some errors. Even seemingly trivial maintenance fixes are likely to be defective.
It's your job as a reviewer to find as many of these defects as possible. If you're not finding defects, you're wasting your time on reviews.
That "one simple step"? Remind yourself that there are almost certainly defects in the work product you're reviewing, and then find them. It's all about attitude.