Underneath - S5-E17
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."
Hell Bound - S5-E4
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: Because they were both friends with Glenn Quinn, David Boreanaz and Christian Kane were uncomfortable filming the Lindsey-posing-as-Doyle arc.
Trivia: David Boreanaz was suffering knee problems during production. To keep him off his feet, a lot of his scenes were shot with him sitting down.
Trivia: The famous "Wilhelm scream" makes an appearance - the first demon Lindsey cuts up cries it out.
Smile Time - S5-E14
Trivia: Gregor Framkin, the creator of the "Smile Time" show-within-a-show, is played by executive producer David Fury.
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: Angel's plan to kill the ranking members of the Circle of the Black Thorn bears a striking similarity to the mass murders ordered by Michael Corleone in the climax of The Godfather and its sequels.
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.
The Cautionary Tale Of Numero Cinco - S5-E6
Trivia: When Carlos flies through the air after being attacked at the start of the episode, he emits a Wilhelm scream.
Answer: Everybody including Angel saw her. After helping them and then being left alone to talk to Angel, Cordelia says that the Powers That Be let her come back one more time to help them. After Angel gets a call saying that she died, he realises that it was her spirit that was helping them the entire time.