Don't quite see using index as key has problems?

On page https://reactjs.org/docs/reconciliation.html
it is said that

Here is an example of the issues that can be caused by using indexes as keys on CodePen

How can the issue be created? I added record to the front, to the back, and sort them, and they seemed to work ok?

Actually, here is a simple case and I can't break it: https://stackoverflow.com/questions/60283967/in-reactjs-using-array-index-as-key-is-said-to-not-work-well-but-i-cant-make

I can only break it if it is all 3 conditions:
It has to be function component using useState, and I have to mutate the state (which the docs says I shouldn't do), and use index as key. So if I don't mutate the state, I can't make it not work well if I use index as key.

Comienza con la página de documentación sobre reconciliación y su ejemplo de CodePen enlazado; después, compara el comportamiento descrito con el caso enlazado de Stack Overflow. Determina si el ejemplo o la explicación necesitan aclaraciones, y considera completa la documentación cuando explique correctamente un caso reproducible o por qué el ejemplo actual sigue siendo válido.