If it's not relevant you may miss it. This phenomenon of inattentional blindness is well documented; in a classic study, most observers asked to monitor a video of a ball game missed a gorilla on the court. But would experts also miss a gorilla? Yes, according to a study on the subject (T. Drew et al. Psychol. Sci. http://doi.org/m9s; 2013).
The authors asked 24 radiologists to search for lung nodules (small, bright circles) in several chest scans; the final scan (pictured) contained the image of a gorilla, 48 times larger than an average nodule. Surprisingly, and perhaps worryingly, 83% of the experts missed the gorilla. It is somewhat reassuring that the miss rate among non-medical observers performing the same task was even higher.
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Shadan, S. Spot the gorilla. Nature 500, 36 (2013). https://doi.org/10.1038/500036a