I'm thinking of this example, where it's debated whether creating a mutable reference to uninitialized values is considered UB (even though it currently is seems de-facto sound to do (if you don't read from it), but not guaranteed (yet, I guess)).
So Rust could declare that creating a mutable reference to uninitialized values is undefined behavior even if it doesn't cause any problems for now.
P.S.: Maybe the linked example isn't the best example for what I meant, because there it's debated whether the "undefinedness" is lifted (or even has been de-facto lifted), which is more of an unclear state rather than one of the three cases I listed.