Love Hurts - S1-E21
Continuity mistake: In this episode Leo specifically tells Prue, Piper and Phoebe not to touch the Darklighters arrow because it is poisonous however in future episodes/seasons, the Darklighter arrows are only poisonous to Whitelighters. (00:06:30)
Love Hurts - S1-E21
Continuity mistake: When Prue shoots the Darklighter arrow at Alec, he orbs out and the arrow sticks into the wall behind him. In the next shot, the arrow has disappeared. (00:25:45)
Love Hurts - S1-E21
Revealing mistake: After Piper saves Leo, they meet with Phoebe and Daisy downstairs near the living room. Before the darklighter appears, smoke from a fog machine can be seen at the left of the shot to magically transition to the computerized effects. (00:33:45)
Love Hurts - S1-E21
Continuity mistake: Leo dies and then Piper heals him. In all future episodes it's made very clear that Leo's powers can't heal the dead.
Answer: "Hush, Hush, Hush" Paula Cole.