Lies My Parents Told Me - S7-E17
Corrected entry: In the episode 'Lies My Parents Told Me' Spike and Dru are waiting for Spike's mother when the enters her living room, but how did they get into the house? Even entering his former home a vampire needs an invitation, and since she was surprised to see them she couldn't have been the one who let them in.
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: If no living person claims ownership of the house then vampires can enter as they please. Seeing Spike with his mother it is probably safe to assume that it was his house and she was a long term guest not an official resident.