In a more helpful framing: safe Rust code doesn't need to worry about its own correctness, it just is.
Unsafe code can be incorrect (or unsound), and needs to be careful about it. Part of being careful is that safe code can call the unsafe code in a way that triggers that unsoundness; in that way, safe code can cause undefined behaviour in unsafe code.
It's not always the case that this is possible; there are unsafe blocks that don't need to depend on safe code for its correctness.
