How embarrassing!
A phrase used to humiliate someone who has just made a mistake or revealed some embarrassing thing about themselves. Is best spoken in a deep voice and used on someone who is giving some form presentation or at least standing in front of a large group of people.
Teacher: (to class) "sorry, i made a mistake up on the board"
Student: (to teacher) "How embarrassing!"
Student: (to teacher) "How embarrassing!"