When Faith (in Buffy's body) calls to get a plane ticket the credit card expiration date is 5/01. This may be foreshadowing of the events that will take place in May of 2001 in the episode "The Gift" (5x22).
Goof: When Giles, Willow, Tara and Buffy-in-Faith arrive at the church and a police officer argues with them, Willow says to him, "But you don't understand" While she is saying this her lips are not moving at all.
Goof: When they are taking Buffy-in-Faith to the ambulance in the beginning and we look from her point of view at Joyce and Faith-in-Buffy. But Buffy-in-Faith's face is facing upward - meaning her pov would be strictly the stars - there's no way she could see Joyce and Faith-in-Buffy, maybe their heads.
At the end of "This Year's Girl" when Buffy and Faith fight in Buffy's house, Buffy has straight hair. In "Who Are You?", when the cops come to get Faith, Buffy has wavy hair now. The end of "This Year's Girl" and the beginning of "Who Are You?" is the same scene.
Giles: Who's president?
Buffy-in-Faith: We're checking for Buffy, not a concussion.
Faith-in-Buffy: So you guys been hanging out a lot lately, huh?
Tara: Yeah, she's really cool.
Faith-in-Buffy: (smiling) So Willow's not driving stick anymore. Who would have thought?
Giles: If you are Buffy, then you'll let me tie you up... without killing me... until we find out whether you're telling the truth.
Buffy-in-Faith: Giles, Faith has taken my body, and for all I know she's taken it to Mexico by now. I don't have time for bondage fun.
Willow: We'll get together with Buffy another time. Sometime soon. I think you'll really like her.
Tara: She's not your friend.
Willow: I may have overestimated the "you liking her" factor.
Riley: Door's open.
Riley: So my fantasies don't tend to include a bunch of Marines staring in at me.
Buffy: How did you respond so fast?
Riley: I didn't. I was just late for church.
Giles: It's about Faith, not surprisingly.
Faith-in-Buffy: Didn't Joyce tell you? I already kicked that ass.
Xander: I feel a high-five coming on.
Spike: You know why I really hate you, Summers?
Faith-in-Buffy: 'Cause I'm a stuck-up tight-ass with no sense of fun?
Spike: Well, yeah, that covers a lot of it.
Faith-in-Buffy: 'Cause I could do anything I want, and instead I choose to pout and whine and feel the burden of Slayerness? I mean, I could be rich, I could be famous, I could have anything. Anyone. Even you, Spike.....I could ride you at a gallop until your legs buckled and your eyes rolled up. I've got muscles you've never even dreamed of. I could squeeze you until you popped like warm champagne, and you'd beg me to hurt you just a little bit more. And you know why I don't? Because it's wrong.
Tara: I am, you know.
(Tara turns and looks up at Willow)
Tara: W-Willow said he w-w-w-w-w-we....
Faith-in-Buffy: w-w-w-w-w-what? You gonna get that sentence out sometime tonight?
Buffy-in-Faith: Oh, when I had psychic power I heard my mother think that you were like a stevedore during sex. Do you want me to continue?
Giles: Actually I beg you to stop.
Buffy-in-Faith: What's a stevedore?
Anya: We were going to light a bunch of candles and have sex near them.
Faith-in-Buffy: Well, we certainly don't want to cut into that seven minutes.
Xander: I believe that's my 'hey.' Hey!
Giles: Look, I know what you are going to say, and...
Buffy-in-Faith: I'm Buffy.
Giles: All right, I didn't know what you were going to say, but that doesn't make you any less crazy.
Faith-in-Buffy: You can't do that - it's wrong - I'll kick your ass.
Faith-in-Buffy: Spike, Spike... William the Bloody with a chip in his head. I kinda love this town.
Sarah Michelle Gellar, in an interview with Entertainment Weekly she gave shortly after announcing her decision to leave the show, claimed that this episode is one of her all time favorites because "I thought Eliza [Dushku] was so great."
The nurse in "This Year's Girl" apparently was calling the Watcher's Council.
Riley attends church. This is the first time we've seen a regular character do so.
Willow: You didn't sense a hyena energy at all, did you? Because hyena possession is just... unpleasant.
This refers to Xander being possessed by hyena in " The Pack" (1x6).
Faith-in-Buffy: Why, yes, I would be Buffy. May I help you? Buf-fy. You can't do that - it's wrong. You can't do that because it's naughty. Because it's wrong. Because it's wrong. You can't do that. It's wrong, I'll kick your ass. I'm gonna kill you.
This bit of Faith in front of the mirror pays homage to Robert DeNiro in front of the mirror in the 1976 film Taxi Driver. In both cases we see a character trying to adopt a new personality at odds with their previous one and get used to it. And in both cases the speech ends with a line about death, both for enemies and for the person whom the character used to be.
Spike: But until then, I'm just as helpless as a kitten up a tree.
This is a reference to the opening line of the song "Misty", which was made famous by Frank Sinatra.
Willow: You're Buffy. You and Faith switched bodies, probably through a Draconian Katra spell.
In the Star Trek franchise of films and television series, Katra is the Vulcan word for soul or spirit.
Buffy: When I had psychic power, I heard my mom think that you were like a stevedore during sex.
Stevedore is a Spanish derivative word for someone who works at a dock loading and unloading ships.