Buffy The Vampire Slayer S06E16

Hell's Bells

Before Xander and Anya's wedding, Xander's future self suddenly appears, and warns him that his marriage to Anya will be the beginning of a life of disgrace and pain. Xander is shown flashes of the future, convincing him to call it off, but later it is discovered it is only a demon taking revenge on Anyanka from her demon days. Nevertheless, the wedding doesn't go ahead and Anya and Xander break up.