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.

reconciliation のドキュメントページと、そこからリンクされている CodePen の例から始め、報告された動作をリンクされている Stack Overflow のケースと比較します。例または説明に明確化が必要かどうかを判断し、再現可能なケース、または現在の例が引き続き有効である理由を正確に説明できていれば、ドキュメントは完成したものとみなします。