You can miss the obvious when your attention is busy.

Inattentional blindness happens when attention is so focused that obvious unexpected objects can go unnoticed.

Inattentional blindness or perceptual blindness occurs when an individual fails to perceive an unexpected stimulus in plain sight because their attention is focused elsewhere, rather than any vision defects or deficits. When it becomes impossible to attend to all the stimuli in a given situation, a temporary "blindness" effect can occur in which individuals fail to see unexpected but often salient objects or stimuli. Inattentional blindness is similar to change blindness in which the observer fails to notice changes that occur slowly in a scene.