Holly Marie Combs and Jaime Gomez previously worked together on an episode of Picket Fences titled "This Little Piggy"
When the Seer hands Phoebe the tonic and says, "Here drink your tonic.", the glass is practically full. In the next scene we see Phoebe handing the glass back to the Seer, but now the glass is empty. We never actually see Phoebe drink the tonic.