When something terrible surprises you when it turns out to be mediocre, thereby juxtaposing the aforementioned terrible something making it appear brilliant.
Susan Boyle is a fugly man-woman. Her singing voice, however, is very average. Because she is so damn ugly, this surprised everyone into thinking her singing voice was angelic. It isn't.
Steve: The Wizard of Oz was a great film...
James: No it isn't. It just appears that way because it was made in a period when all other films were in black and white.
Steve: Aah, the Susan Boyle Effect!