Ground State - S4-E2
Revealing mistake: In 'Ground State' when Gwen is walking towards the lift in Irwin's building you can see Angel's reflection on the shiny door. Vampires don't have reflections, as is demonstrated at various other times in this show.
Ground State - S4-E2
Factual error: In the flashback to Gwen as a child in 1985, she is dressed in a jacket tagged with an "Old Navy" zipper. That clothing store chain wasn't founded until 1994.
Revealing mistake: When Gunn, Fred and Lorne run out to the Las Vegas strip, a large crowd of bystanders can be see seen in the background watching the show being filmed (as said in the DVD commentary). (00:28:25)
Continuity mistake: When Angel is talking to Cordelia in the garden at the very end of the episode "Supersymmetry" he has a cut on his cheek. The beginning of "Spin The Bottle" is the continuation of that scene, but Angel's face no longer has a cut.
Continuity mistake: After Cordelia kisses Connor, her hand is on his cheek. Just before she says "and if this is the end", in the shot of Connor she removes her hand, then the angle changes to show her, and her hand's suddenly back on his cheek again.
Continuity mistake: When Angel grabs Cordelia's hand as she apologies after telling him to "get over it", the position of Angel's hand changes between shots.
Other mistake: The Chinese writing shown tattooed all over Wo-Pang's body is nonsense (a small number of random characters repeatedly jumbled together), although Wesley reads it as if it is language. (Although one might argue that the writing is meant to be some sort of code, there is no evidence in the show itself to suggest that, and Wesley refers to "translating" rather than "decoding" it; for these reasons it is more likely to be simply a mistake rather than an intentional decision.) (00:16:40)
Continuity mistake: When Cordelia looks at the monitor, the reflection of Angelus's cage shows Angelus standing at the bars next to Connor. It cuts to a direct shot, and Angelus is sitting on the ground on the other side of the cage. (00:21:40)
Audio problem: During Lilah's failed attempt to rescue Angelus, when Gunn shoots at Angelus with the tranquilizer gun, Angelus passes out then hits the ground. When he is about to pass out, we hear the sound effect of him hitting the ground before he even begins to fall over, and instantly in the next camera angle he hits the ground. (00:08:20)
Continuity mistake: When Wesley tells Faith that Angelus is back, she says, "Step away from the glass" and hangs up the phone. When Wesley jumps from his chair (to allow Faith to smash through the glass and escape) you can see Faith behind the glass with her hands up to her face and standing, but in the close up of Faith her hands are at her sides and she is only just getting out of her chair. (00:23:20)
Continuity mistake: When Cordy throws the knife at the door after Willow leaves, the knife hits at an impossible angle from where it was supposed to have been thrown.
Audio problem: When Gwen says to Garret, "I know the risk, but I'm going in there with or without your help," her lips keep moving as if she's still speaking.
Inside Out - S4-E17
Continuity mistake: The sketch of Cordelia that Angel looks at depicts her facing to her left. But once he places it back on Wesley's desk, it depicts her facing to her right, revealing a flipped shot.
Answer: Due to his epiphany, Angel basically had the 'true darkness' he worked so hard to reattain ripped away from him, like his soul was after sleeping with Buffy.