Trivia: In the episode "Underneath", Illyria is talking about different dimensions she has visited and she says "And one world with nothing but shrimp...I tired of that one quickly." This is a reference to Buffy episode "Superstar": when Anya is explaining that there are many types of alternate realities, she says, "And there could even be a world with nothing but shrimp."
Trivia: Angel leaves the Reaper locked in a metal cage as the camera pans slowly back so we can only see his eyes - almost exactly the same shot as when Connor left Angel trapped in the metal coffin at the end of "Tomorrow". They even say the same thing - "You get to live. Forever". Like father, like son.
Trivia: When the gang is trying to figure out how Illyria got by them, Gunn says "Yeah, like she was pulling a Barry Allen. Jay Garrick? Wally... Like she was moving really fast." No one knew who he was talking about. All three name are different incarnations of The Flash.
Trivia: With this episode, David Boreanaz is the only remaining original cast member from season one. Glenn Quinn's character died in episode 9 (He died in real life 3 years later) , Charisma Carpenter was written out of the show at the end of season four, while Alexis Denisof and J. August Richards began as guest stars; Amy Acker and Andy Hallett weren't introduced until season two, and Vincent Kartheiser first appeared in season three.
Trivia: Joss Whedon wanted Sarah Michelle Gellar to guest star as Buffy in "You're Welcome" (the show's 100th episode) and "Power Play" (the penultimate episode), but she was unavailable. She later became available for the series finale, but Whedon declined, wanting the series to end focused on Angel. Conversely, David Boreanaz was available for the Buffy finale, but only for a limited amount of time.
Trivia: When the gang are in Spike's apartment where Angel is assigning them to go after members of the Circle of the Black Thorn, Spike says "First off, I'm not wearing any amulets. No bracelets, broaches, beads, pendants, pins, or rings. " This is a reference to how Spike dies in the series finale of Buffy by wearing an amulet to destroy the Hellmouth.