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.

Commencez par la page de documentation sur la reconciliation et l’exemple CodePen qui y est associé, puis comparez le comportement signalé avec le cas Stack Overflow associé. Déterminez si l’exemple ou l’explication nécessite des clarifications, et considérez la documentation comme complète lorsqu’elle explique correctement un cas reproductible ou pourquoi l’exemple actuel reste valide.