When Willow feels that no one cares for the pain she feels for Oz leaving, she casts a spell so to make her will reality.  After thinking that the spell failed, she inadvertantly wishes that Buffy and Spike get married, Giles goes blind and Xander to be a demon magnet.  Willow is confronted by D'Hoffryn who offers to make her a vengeance demon  but, when she sees what she's done, she refuses and reverse the spell.