Continuity mistake: Willow is turned into a ghost. Throughout the entire episode, she can't touch anything (can't turn the pages of a book, etc.), yet towards the end when she and Giles go to Ethan's shop, and she leaves, you can hear that she opened and closed the door, even though she can walk through walls and can't actually touch the door. The curtain moves when she leaves too.
Buffy's prophetic dreams gave several hints about Dawn long before her arrival. In 'Graduation Day, Part 2', season 3, dream Faith tells her 'little sis' is coming', and in 'Restless', season 4, dream Tara warns her 'Be back before Dawn'.
There have been several lines throughout the series have referred to Tuesday ('There's a demon trying to destroy the world and it's just another Tuesday night in Sunnydale' in Revelations and 'Dawn's in trouble. Must be Tuesday' in Once More With Feeling). These are references to the fact that, in the USA, Buffy is aired on a Tuesday.
Spike: You listen to me. [Kneels in front of her.] I've been alive a bit longer than you, and dead a lot longer than that. I've seen things you couldn't imagine, and done things I prefer you didn't. I don't exactly have a reputation for being a thinker. I follow my blood, which doesn't exactly rush in the direction of my brain. So I make a lot of mistakes, a lot of wrong bloody calls. A hundred plus years, and there's only one thing I've ever been sure of: you. [Buffy looks away; he reaches toward her face.] Hey, look at me. I'm not asking you for anything. When I say, "I love you, " it's not because I want you or because I can't have you. It has nothing to do with me. I love what you are, what you do, how you try. I've seen your kindness and your strength. I've seen the best and the worst of you. And I understand with perfect clarity exactly what you are. You're a hell of a woman. You're the one, Buffy.
Buffy: [Quietly.] I don't wanna be the one.
Easter egg: On the second DVD from select the episode 'Once More With Feeling.' In the following screen go to the 'Language Selection' and there highlight the menu entry 'English for the hearing-impaired.' Now press the 'Left' arrow key on your remote control and the Buffy 'B' will be highlighted giving you access to 3-minutes worth of footage from the soundtrack signing at Tower Records in Los Angeles. On the fifth disc select the episode 'Normal Again' and then go to the 'Language Selection.' Highlight 'English for the hearing-impaired' and then press the 'Left'. You will now see a sign saying that there is a 'Computer Download' available for the episode call sheet.