Continuity: Phoebe is superstitious. In "The One with All the Wedding Dresses" she believes that on the wedding day, the groom should not see the bride before the wedding. However, on her own wedding day, Phoebe and Mike hang out together before the ceremony.
Note: Many would argue that the groom isn't supposed to see the bride in her wedding dress before the ceremony, but otherwise it's okay. In the "Wedding Dresses" episode, when Rachel is about to answer the door and Phoebe cautions her about it, Rachel is wearing her wedding dress. In "The One with Chandler and Monica's Wedding (2)", Judy says about Chandler, "Well, he better not come by here, he can't see the bride in the wedding dress...".