Here is a commonly observed effect: Some things are very hard to find because they're embedded in surroundings where they don't stand out, but after you've found one, it's easy to find it again—and it's hard to perceive the original difficulty of finding it.
For example, finding Waldo on a page of a Where's Waldo? book is usually very difficult. You have to search through many "decoy" images on the same page. But once you've found Waldo on that page, finding him again is much easier. You can omit nearly the whole page from your search, and Waldo visually "pops out" more easily. Sometimes you can't not see Waldo.
- Solving a puzzle or brainteaser question is usually difficult, but once you know the solution, solving it again is easy.
- Learning mathematics is usually difficult, but often after you've learned a piece of it, e.g. the Fundamental Theorem of Calculus, it seems almost trivially obvious.
- Learning to perceive spoken words in a foreign language is usually difficult at first, but once you've got it, it's effortless; you can't even not perceive them.
Usually after learning one of these things, one tends to underestimate its difficulty for someone who hasn't solved it. For example, after finding an outdoor sign for a business on a crowded street, people will sometimes point and say, "It's right there—the yellow sign! Can't you see?," forgetting how hard it is for someone who hasn't seen the sign yet to see it.
You might reasonably say that the above examples illustrate several distinct effects, not just one effect. That's OK. If there are multiple terms or multiple forms of "Where's Waldo?" effect distinguished in the literature, I'd love to see an answer with more than one. I'm mainly asking in order to get terms to search the literature on Google Scholar. I'm as interested in the base effect as in the difficulty empathizing with people who haven't yet "found Waldo".