Jared Padalecki (Sam Winchester) dislocated his shoulder while wrestling with Osric Chau (Kevin Tran). The writers were then forced to make up a story as to why Sam was wearing a sling in the beginning of season ten. None of the cast or crew members ever expected the show to last as long as it had.
Sam's supernatural abilities are the result of him being fed Azazel's demon blood while he was an infant. Sam exhibits signs of precognition throughout the first season, manifesting as dreams of others' deaths and later as visions.
