When Willow and Xander first enter the library, there are two paper-trays on top of the catalog cabinet where Buffy is standing; they are absent in subsequent shots. Also on the cabinet is a chained pen, which changes position between shots.
At about 24:01 Buffy calls Willow in alarm. Willows hair is over her right shoulder. Cut to Buffy. Back to Willow all of her hair is now behind her back. Each exchange moves Willow's hair from in front of her shoulder to behind it throughout the scenes.