The fourth key to effective software testing deals with the common experience of overlooking things that can "fall through the cracks." The simple but not always easy way to reduce such oversights is to follow software testing guidelines that help the tester be more thorough. Software testing guidelines include checklists and templates meant to guide development or testing.
Consider the difference between going to the supermarket with and without a shopping list, which is an example of software testing guidelines. Without a list, you tend to spend more yet come home without some of the groceries you needed. With the shopping list, you get what you need and spend less because you're less likely to make impulse buys.
Software testing guidelines also help detect omissions that exploratory testers are much more likely to miss. By definition, exploratory testing is guided by executing the software as built. That tends to channel one's thinking in line with what's been built, which easily can lead away from realizing what hasn't been built that should have been. Software testing guidelines can help prompt attention to such items that following the product as built can obscure.