For example, accidentally sharing a lock-less cache or a non-atomic reference counted pointer between threads.

For example this code, which tries to send a reference-counted pointer between threads, which can cause the reference counter to become unsynchronized and random use-after-free:

    use std::thread;
    use std::rc::Rc;

    fn main() {
        let rcs = Rc::new("Hello, World!".to_string());
    
        let thread_rcs = rcs.clone();
        thread::spawn(move || {
            println!("{}", thread_rcs);
        });
    }

Is detected by the compiler and causes this error

    error[E0277]: the trait bound `std::rc::Rc<std::string::String>: std::marker::Send` is not satisfied in `[closure@src/main.rs:8:19: 10:6 thread_rcs:std::rc::Rc<std::string::String>]`
     --> src/main.rs:8:5
      |
    8 |     thread::spawn(move || {
      |     ^^^^^^^^^^^^^ `std::rc::Rc<std::string::String>` cannot be sent between threads safely
      |
      = help: within `[closure@src/main.rs:8:19: 10:6 thread_rcs:std::rc::Rc<std::string::String>]`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<std::string::String>`
      = note: required because it appears within the type `[closure@src/main.rs:8:19: 10:6 thread_rcs:std::rc::Rc<std::string::String>]`
      = note: required by `std::thread::spawn`