The following deserves no unchecked diagnostic, but javac produces one anyway.

interface A<T> {}
class B<T> {}

class C {
        A<?> x = null;
        B y = (B)x;
}
Another test case reported by the Eclipse team

import java.util.*;
public class X {
    X(List rawList, List<?> unboundList) {
            Throwable t0 = (Throwable) Collections.emptyList(); // unchecked
            Throwable t1 = (Throwable) rawList; // no warn
            Throwable t2 = (Throwable) unboundList; // unchecked
            Object o = unboundList;
            Throwable t3 = (Throwable) o; // no warn
    }
}
Another test case clearly showing the silliness of the warning:

public class C {
    interface I<T> {}
    static void m(I<?> o) {
        System.out.println("checking " + o);
        System.out.println("[#1] instance of Exception? " + (o instanceof Exception));
        System.out.println("[#2] instance of Exception? " + Exception.class.isInstance(o));
        System.out.println("[#3] instance of Exception? " + ((I) o instanceof Exception));
    }
    public static void main(String[] x) {
        class I1 implements I<String> {}
        m(new I1());
        class I2 extends Exception implements I<String> {}
        m(new I2());
    }
}

Though all three checks do the same thing, line #1 elicits an unchecked warning from the compiler. Note that checking 'o instanceof Cloneable' (an interface) results in no warning.