In the paper, we provide several thoughts, including one example, when type _in_stability actually makes sense. In particular, the example concerns a typical `parse` function, that converts a string into an AST node. Assuming you have one struct per AST type, the parse function is unstable.

Generally, several well-known OOP design patterns would naturally lead to instability. In turn, those patterns can be handy to solve certain typical problems. If you have such a problem at hand, you have a choice: either do a well-known thing and embrace instability (which is actually not always all that harmful for performance), or get out of your way and torture your code to make it stable…