class Program
    {
        static void Main(string[] args)
        {
            new B().Test().Count();
        }
    }

    class A<T>
    {
        public virtual IEnumerable<T> Test()
        {
            yield return default(T);
        }
    }

    class B : A<int>
    {
        public new IEnumerable<int> Test()
        {
            base.Test();
            yield return 0;
        }
    }

BadImageFormatException -> An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)
Обсуждение.