This compiles but should not. Running produces a CCE.

public class WildSubtyping {
    static class Container<T> {
        T value;
    }

    static class A<T> {
        T aValue;

        void putT(T t) {}
    }

    static class B<T> extends A<A<T>> {
        T bValue;

        @Override
        void putT(A<T> a) {
            bValue = a.aValue;
        }
    }

    static void bar(Container<? extends A<A<?>>> container) {
        A<Integer> a = new A<Integer>();
        a.aValue = 4;
        container.value.putT(a);
    }

    static void foo(Container<B<?>> container) {
        B<String> b = new B<String>();
        container.value = b;
        bar(container); // Container<B<?>> is not subtype of Container<? extends A<A<?>>> !!
        b.bValue.length();
    }

    public static void main(String[] args) {
        foo(new Container<B<?>>());
    }
}

Reported by Ross Tate, as encountered by the Kotlin team.