The Rust Programming Language Standard Library 1.34.x before 1.34.2 contains a stabilized method which, if overridden, can violate Rust's safety guarantees and cause memory unsafety. If the `Error::type_id` method is overridden then any type can be safely cast to any other type, causing memory safety vulnerabilities in safe code (e.g., out-of-bounds write or read). Code that does not manually implement Error::type_id is unaffected.
External References:

https://blog.rust-lang.org/2019/05/13/Security-advisory.html