What's special about UB is that it attacks your ability to find bugs, like a disease that attacks the immune system. Undefined behavior can have arbitrary, non-local and even non-causal effects that undermine the deterministic nature of programs. That's intolerable, and that's why it's so important that safe Rust rules out undefined behavior even if there are still classes of bugs that it doesn't eliminate.
@trentj in Newbie learning how to deal with the borrow checker - #11 by trentj