Parameterized type keys for Map

I am learning Java and am currently reading Joshua Bloch's Effective Java.

In Item 29, he discusses parameterized type keys for Map to create a type-safe heterogeneous map. Here is the code:

class Favorites {
    private Map<Class<?>, Object> favorites = new HashMap<Class<?>, Object>();

    public <T> void putFavorite(Class<T> type, T instance) {
        if (type == null)
            throw new NullPointerException("Type is null");
        favorites.put(type, instance);
    }

    public <T> T getFavorite(Class<T> type) {
        return type.cast(favorites.get(type));
    }
}

He goes on to say that

A malicious client could easily corrupt the type safety of a Favorites instance, simply by using a Class object in its raw form. But the resulting client code would generate an unchecked warning when it was compiled.

I know that Class<T> would be erased to Class. But I am not sure how a malicious client can break the type safety at compile time. I tried various ways, but I always got a compiler error, as I expected.

Can someone please tell me what exactly Joshua Bloch meant in the line quoted above?