When Spike heads off to fight the demon, he stops by the high school basement to retrieve his infamous leather coat out of a box of stuff. But there's a problem. The last time we saw Spike's coat was in Seeing Red (2002) (Season 6) when he went to Buffy's house and almost raped her. Buffy throws Spike out, but he leaves without his coat. When Xander gets to Buffy's he knows Spike has been there because he finds Spike's leather coat hanging over the stair railing. Spike leaves on his journey to get his soul and his coat stayed behind at Buffy's. He and Buffy never saw each other again until he was already living in the basement and talking to ghosts. So when did he get his coat back to box it up and put it in the basement?
When Buffy emerges into the desert she has different boots than in the house. In the house she had spike heels, in the desert she has chunky heels.
When Buffy returns from confronting the men who created the First Slayer, the green salt circle that Willow created on the floor is completely gone.
During the story of the first slayer, the words in Dawn's book magically translate themselves into English, as she tells Xander "it is not Sumerian anymore". Actually, the text is in Greek.
The face of the stunt double is briefly visible when the "exchange demon" throws Xander across the room, breaking furniture as he lands.
When Spike tells Principal Wood that his soul is a work "in progress," he pronounces it as a Californian would, "Prah-gress," rather than as a Londoner would, "Proh-gress."