If you render
<div>
<span key={1}>hello,</span>
<span>world</span>
</div>
then React throws a duplicate-key error because the second span ("world") is assigned a key of 1, its index in the div's children.
Presumably it's best to simply avoid specifying only some keys but it would be nice if React could use separate namespaces for different key types (or at least provide a slightly more informative error message).