Conversations With Dead People - S7-E7
Corrected entry: When Willow is talking to Cassie in the UC Sunnydale University Library, Cassie said she was sent to talk to her by Tara. Cassie says "she still sings the song you sang to each other on the bridge", referring to "Once More with Feeling". But Willow only sang 2 lines in the that episode. In "I've Got a Feeling", she sang "Ive got a feeling some kid is dreaming and were all stuck inside this wacky broadway nightmare", and in "Walk Through the Fire", she sang "I think this line's mostly filler".
Correction: Willow & Tara sing "Under your spell" in a park somewhere in Sunnydale. Part of the song takes place on a bridge going over a pond.