I know about that (even though it's a worthwhile addition to this thread for other readers), but I don't think it's what they had in mind, since they were talking about “Rust devs get bitten by this all the time when trying to reuse "safe" library code within an unsafe context” in their original comment[1], and I really can't see what they are talking about except some variation around “from C++, I passed some uninitialized memory to a Rust library and Rust went boom” but maybe I'm just misunderstanding.
Isn't it easy to call safe functions from unsafe Rust that, if called from safe Rust, would lead to a compilation error? For example, accidentally passing a safe function two mut pointers to the same object, which normal Rust ownership wouldn't allow you to?
Technically speaking it is OK to pass two aliasing mut _pointers_ to a (safe) Rust function because safe Rust can't do anything dangerous with raw pointers; both dereferencing and writing through a raw pointer require unsafe. If we are talking about references instead, creating two mutable references to the same object in Rust (unsafe or not) immediately causes UB.
I can't think of any such thing, no. Unsafe Rust doesn't turn off the borrow checker, it just lets you do a handful of operations that you can't do otherwise. The only way I could see that happening would be if you somehow violated ownership invariants in the unsafe block, which is already forbidden.
[1]: https://news.ycombinator.com/item?id=32878775