Revealing mistake: In the opening scene, where Caleb picks up the potential slayer in his truck, there is a brief shot of the two from the hood that pans slightly to the right. In doing so it reveals the edge of a screen showing a fence and trees passing by, revealing that the car is in fact stationary and in a studio.
Revealing mistake: The entire scene where Buffy is experiencing accelerated time, shadows of trees move in the wind but never actually grow or shrink. Even over short increments of time, the sun can move shadows noticeably, and judging from the speed of the people around her, Buffy is in accelerated time for at least half an hour.
Continuity mistake: In the scene where Xander and Willow are stuck in the factory Xander wakes up you'll notice that the blood on his side burn closest to his ear constantly changes lengths from shot to shot. It goes from under the chin to slightly below where the blood on the other side of his side burn is.
Continuity mistake: In the scene where Anya is undressing a sick Xander in his basement, she lifts up his shirt and says 'Help me get your trousers off.' During the following conversation, in the close-up shots his shirt is lifted far up on his chest, but in the wider shots it's covering his whole stomach.
Continuity mistake: When Anya is confronting Dawn about Xander's disappearance and the demon boy leaves, he walks past Dawn and her attention is on Anya but in the next shot you see him walking away from behind and his sleeve is still in the shot as Dawn turns back to Anya.