“Black Sunday”
In Super Bowl XIII between the Pittsburgh Steelers and Dallas Cowboys in 1979, the Steelers opened as 3.5-point favorites, but money poured in on Pittsburgh, resulting in a closing spread of -4.5. Final score of that game: Steelers 35, Cowboys 31.
It was a devastating result for books that took significant amounts of money on both Pittsburgh -3.5 and, later, Dallas +4.5.