When Angel gets the call about Buffy, Spike asks: "What is it this time? Uber-vamps, demon-gods, devil robots?" The first of those is a reference to Buffy's season seven, in which they were part of the First's army. They have never appeared on Angel, only the other two have (Illyria and the cyborgs from "Lineage" respectively).
Spike's jacket gets blown to pieces in this episode. He gets another 10 from Bianci of Wolfram & Hart Rome, but its not the same exact jacket.
Andrew makes it very clear at the end of "Damage" that he and Scoobies think that Angel is evil and yet he has no problem inviting him into his apartment? He's not even worried that the now "evil" Angel would be angry that Andrew double-crossed him?
Judging by the blast zone after the bomb explodes there was no way Spike and Angel could have survived it at that range. Even if you ignore the fact that the two of them should have caught fire from the blast, and then died from that, the blast would have blown them apart. You could argue that vampires have more resilient bodies than humans and that's how they survived, but as Spike was holding the bomb he certainly should have at least lost his arms (again)!
Spike says he's been wearing his leather jacket for over 30 years. But he got it in 1977, which was only 27 years before this episode aired.
Angel asks the female Italian bartender at the club in his own words, "We're looking for a girl, American, blonde hair, blue eyes." referring to Buffy Summers.
Wesley: Stop it! Change back. Be blue. Be anything. Don't be her. Don't ever be her.
Illyria: As you wish.
Angel: Gunn, we didn't get the head. Start preparing the troops for war.
Gunn: The head is here.
Gunn: The head's on your desk.
(Angel reads the note with it)
Angel: "With regards, the Immortal." I really hate that guy.
(Spike pulls up on a motor scooter to Angel)
Spike: Hop on, little mama.
Angel: I'm not riding on the back.
Spike: He's getting away!
Ilona Costa Bianchi: Now, what happened to the drop? No grazie, prego, kiss, kiss?
Angel: Grazie. Prego. Kaboom.
Ilona Costa Bianchi: The gypsies are filthy people. (spits) And we shall speak of them no more.
Angel: Ours is a forever love.
Spike: I had a relationship with her, too.
Angel: Okay, sleeping together is not a relationship.
Spike: It is if you do it enough times.
Spike: I don't even speak the language.
Angel: We'll get you a book.
Spike: How do you say "wank off" in Italian?
Gunn: Will someone just get on a plane?
Angel: (about the little bottles of alcohol on the plane) Huh. Really can't get drunk off these things.
Spike: Not us, anyway. Vampire constitution not always a plus. How did you know?
Angel: Drank a lot of 'em, and I still don't like you.
Spike: About Buffy. How did you know she was in trouble?
Angel: I got word.
Spike: From who?
Angel: A source.
Spike: You've been spying on her?!
Angel: I just wanted to make sure she was all right.
Spike: Sending your lackeys to do your stalking for you. That is really pathetic.
Angel: All right, fine. I'm not proud of it, but it's... it's Buffy.
Spike: How many you got on her?
Angel: Uh, just the one. But he got spotted. Called me from the hospital
after he regained consciousness.
Spike: Every time he shows up, I either lose my girl, get beaten by an angry mob, or get thrown in prison for tax evasion. (Angel stares) Long story.
Angelus: That's why he had us tossed. So he could violate...
Darla: He didn't...
Angelus: Violate our women!
William (Spike): Violate in succession!
Angelus: Concurrently? You never let us do that.
Spike: I just want to see you happy. Not too happy though or I might have to stake you. Second thought...have at it.
Angel: Our friend, she's under some sort of spell...
Spike: Cast by the vilest wretch this side of Mount Everest. Which...I'm told he has climbed...several times.
Angel: You want a drink?
Spike: God yes!
Spike: Can't we just... lock her away in a box where no one can ever touch her? You know? Like we did with Pavayne?
Angel: I don't think she'd let us. She's pretty strong.
Spike: We could do a spell. Some sort of mind control.
Angel: Oh, she'd figure it out. You know, she's pretty smart.
Spike: So, what? We just have to live with it? Get on with our lives?
Angel: 'Fraid so.
Spike: Fine. No problem. I was planning on doing that anyway.
Angel: Yeah, me, too.
Spike: Actually, I'm doing it right now. As we speak, I'm movin' on.
Angel: Movin' on.
Angel: Right now.
Angel: But she's not finished baking yet. I gotta wait 'til she's done baking. You know, 'til she finds herself. 'Cause that's the drill. Fine. I'm waiting patiently, and meanwhile The Immortal's eating cookie dough!
Andrew: Um... Spike, is Angel crying?
Spike: No. (looks at Angel) Not yet.
Spike: She's glowing, mate.
Angelus: She isn't.
Darla: Little bit.
Spike: Best fit you for a pair of antlers. Been made the right cuckold, you have.
Drusilla: (steps out) Time for another pony ride?
Spike: Son of a bitch!
Angelus: The both of ya?
Darla: He's insatiable.
Darla: Come on, Dru, let's take a bath.
Drusilla: Will you hold me under the water?
Spike: The Immortal?!
Angel: I mean, come on!
Spike: She's smarter than that!
Angel: She'd never fall for a centuries old guy with a dark past who may or may not be evil.
Demon: Oh, look - the Americans are relying on violence to solve their problems. What a surprise.
Angel: I helped save the world, you know.
Spike: Like I haven't.
Angel: Yeah, but I've done it a lot more.
Spike: Oh, please.
Angel: I closed the Hellmouth.
Spike: I've done that.
Angel: Yeah, you wore a necklace. You know, I helped kill the Mayor, and, uh, Jasmine...
Spike: Do those really count as saving the world?
Angel: I stopped Acathla. That saved the world.
Spike: Buffy ran you through with a sword.
Angel: Yeah, but I made her do it. I signaled her with my eyes.
Spike: She killed you. I helped her. That one counts as mine.
Spike: The man has no sense of indecency. You remember Frankfurt. He hatches the Rathruhn egg personally, and then just decides to give those nuns safe passage.
Angelus: Those were my nuns!
Spike: Yeah. Nuns are your thing. Everybody knows that. They respect it. They respect us.
Angelus: We are the reason men fear the night!
Angelus: Go ahead. Take your best shot. I'll snatch your little wee sticks out of the air and spend the next fortnight shoving them slowly up your arse.
Spike: Can you really do that?
Angelus: The arrow thing? I don't know - never tried.
This was the last time that any character crossed over from Buffy to Angel.
Illyria confirms that she could talk to plants (an ability Spike suggested she might have in "Origin" and "Time Bomb"), and she refers to it as "the song of the green." However, she lost that ability when Wesley drained most of her powers during "Time Bomb," and can no longer hear them.
This episode marks the final appearances of Julie Benz (Darla), Juliet Landau (Drusilla), Tom Lenk (Andrew), Gary Grubbs (Roger Burkle), and Jennifer Griffin (Trish Burkle) on the show.
The song playing during the bar fight is Dean Martin's "Take Me in your Arms."
The Rome Wolfram & Hart looks exactly like the one in L.A.
The techno music playing when Angel and Spike are at the club looking for Buffy is the same that Spike comments on in 'Life of the Party' (5x05). It was one of the songs playing at Lorne's Halloween bash.
Angel: We're gonna need a chase helicopter, assault team...
Spike: And guns. Lots and lots of guns.
In the movie The Matrix, the main character Neo delivers a popular line almost identical to Spike's line here. Like Spike, Neo also frequently wears a leather trenchcoat.
The image on Andrew's T-Shirt is "Strong Bad," one of the main characters featured on the Homestar Runner website.