The [Missing square puzzle](https://en.wikipedia.org/wiki/Missing_square_puzzle) problem is a good illustration. It illustrates a simple and universal problem/question/solution process, where "some illusion" causes complications.

There are a self-evident problem — the 1×1 hole in that arises on the second figure —, but the solution is evident only after we know (like an Egg of Columbus)... All people, experts and non-experts, agree that there is a problem.

![enter image description here][1]

 *  **normal user**: thinks that <br/>"there are two *equivalent* figures, the 'total triangles' in a perfect 13×5 grid...",<br/> following with the question:

    - **problem `Y`**: *"Why does the second equivalent triangle have a hole in it?"*. 

 *  **expert user**: think something like <br/>"oops, they are alike (*close in shape*), but not *perfectly equivalent* figures",<br/> following with the question:

    - **problem `X`**: *"How to show that they are not perfect equivalents?"*.

The clever geometry-expert thinks in terms of *close in shape* geometries.

The normal user thinks, mistakenly, in terms of exact [congruence](https://en.wikipedia.org/wiki/Congruence_(geometry)). So, the  use of **wrong hypothesis**, produce ill `Y` questions.  

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## The "XY problem" as a specialization of the "wrong hypothesis behaviour"

You want to solve the real question-`X`, and you think in terms of an `Y`-context, and try to use question-`Y`. Instead of asking about context `X`, you ask about context `Y`.  <br/>(as [@Gnome noticed above](https://meta.stackexchange.com/a/66378/205446), but using other words)

So, *"XY Problem"* is only another (more specialized) term to say *"Use of [wrong working hypothesis](https://en.wikipedia.org/wiki/Working_hypothesis)"*.

  [1]: https://i.sstatic.net/Mw9gg.gif