Because it was a clown, Sam hesitated out of fear just long enough for Gacy to slash him across the chest.
Unfortunately for poor Sam and his enduring fear of clowns, he happened upon the ghost of John Wayne Gacy when he was attempting to save a mom and her little girl.
While Dean and the demon Belphagor, who was inhabiting the corpse of Jack, were tasked with taking out the Woman in White, Sam and Castiel faced off against some other ghosts.
Cas showed up to save the day, but not before things got pretty frightening.