Perceptional and Inattention Blindness
When people focus on something that demands their attention, they often fail to notice unexpected objects and events that occur in their line of sight. This is known as "inattentional blindness"because people typically do not consciously perceive aspects of their world that fall outside of the focus of their attention. These events can be dramatic enough that the vast majority of people are convinced that they would notice. In reality, though, many people do not. We can easily miss something that should be completely obvious.
"If you don't believe it, you won't see it". Lifeguards who think a drowning will never happen to them or at their pool are at high risk for missing the obvious.