Their issue tracker has a label for soundness holes:

https://github.com/rust-lang/rust/labels/I-unsound

Not a counterexample of the type system of Rust itself, but this one with LLVM following C and C++ definitions of "side-effects" is interesting: https://counterexamples.org/eventually-nothing.html

Don't know what's interesting for you, but this is definitly interesting: https://counterexamples.org/eventually-nothing.html I've also found these https://counterexamples.org/nearly-universal.html https://counterexamples.org/selfishness.html