The Superman Curse refers to a theory that is based on a series of misfortunes that have plagued creative people involved in adaptations of Superman in various media, particularly actors who have played the role of Superman on film and television.
The curse in a nutshell:
If you play the strongest man on Earth, you will either die an untimely death or end up in the weakest position possible.
The curse is somewhat well-known in popular culture, though some would deny the curse, stating that several Superman-related actors, such as Dean Cain, Teri Hatcher, Bud Collyer, and even Brandon Routh, continued with successful careers after their association with the franchise. Another factor that attempts to better explain the curse is the "no cape or on-screen time" rule, meaning that actors of the Smallville TV series have not been cursed due to the Superman outfit not being worn, or that voice actors who have played Superman in radio or animated adaptations, namely Bud Collyer and Tim Daly, have escaped the curse.