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.

Beginne mit der Dokumentationsseite zur Reconciliation und dem darin verlinkten CodePen-Beispiel, und vergleiche anschließend das berichtete Verhalten mit dem verlinkten Stack-Overflow-Fall. Ermittle, ob das Beispiel oder die Erklärung einer Klarstellung bedarf, und betrachte die Dokumentation als vollständig, wenn sie einen reproduzierbaren Fall oder den Grund, warum das aktuelle Beispiel weiterhin gültig ist, korrekt erklärt.