You may have heard about the park ride in Germany, which, when you look it from underneath looks like 4 twirling swastikas. After videos of the ride were put online the ride was shut down and is being redesigned.
After you watch the video you may shake your head and ask yourself "How could they have not seen that?" It has to do with perceptual set.
Recall that famous drawing of the young woman/old woman. If you were shown, say, 5 pictures of a young woman and then were shown the drawing, you would probably see a young woman first. On the other hand, if you were shown 5 pictures of older women and then shown the drawing you would probably see an older woman first. We see what we are primed to see. Or as we say, what we expect to see.
That's what's happening with the amusement park ride. Most likely, before you saw the video you were told that you would see swastikas. You were primed to see swastikas. And so you saw them. If you were not told about swastikas, but instead were told about a "fun new amusement ride" you probably would not see swastikas.
But how could such a ride have been created? Surely the designers of the ride saw the swastikas? Perceptual set theory would say that the designers were thinking all along the creation process that they were building an amusement park ride. Their minds were filled with images of other amusement park rides. So they didn't see what to us seems obvious.
That's one reason why a team that is grappling with a problem might say, "Let's bring in someone with a new perspective". That someone you're looking for is someone who has not been influenced by the current team's perceptual set.
So that drawing of the old woman/young woman isn't just a fun "optical illusion". It's a demonstration of an important way our minds work.