My nomination for Biggest Costume Disaster isn't an original one, but it's very grand:
Kurt Seligman's original designs for the 1946 premiere of Balanchine's The Four Temperaments.
In vintage photos we see Todd Bolender's Phlegmatic looking rather like Marcel Marceau in motley dress, clown makeup, and what looks like a felt top hat that has been through an old-fashioned clothes wringer.
This is based on photos of individuals. I can't imagine a group of these people on stage at the same time.
What are your own favorite costume disasters? You must have seen -- or heard of -- at least a few over the years!