The trophy that Catherine Madison is trapped in (Witch (1997)) can be seen on the floor as the Scoobies walk through the halls.
Much of Anthony Head's dialog has been very noticeably re-dubbed in this episode. This is most noticeable in the scene where Buffy first asks Giles about the earthquake - the lip-sync is off and the audio is far too clean in comparison to Sarah Michelle Gellar's.
Spike learns that his chip doesn't prevent him from hurting demons. This marks the beginning of Spike fighting evil alongside the Scoobies, albeit for different motives.
Buffy explains the difference between herself and Riley is that he treats demon hunting as a job, while for Buffy, it is her destiny. This echoes Buffy's conversation with Kendra at the end of What's My Line?: Part 2 (1997), in which Buffy's role was reversed and Kendra lectured Buffy about not treating slaying like a job because it is who she is. In the two years since that conversation, Buffy's experience has forced her to more readily reconcile her identity with that of being the Slayer.
This is the first time the ruins of Sunnydale High are shown, after the school was destroyed in Graduation Day: Part 2 (1999). During the fight in the library, the book cage can be seen hanging above the demon in one frame. The floor has been blown away, cutting into the earth, making the base of the library lower than that of what it was during the first three seasons