A New World - S3-E20
Revealing mistake: When Connor runs out of the hotel and Groo and Gunn drag Angel out of the sunlight, Angel's reflection can be seen in the door behind him just as Cordelia steps outside. Angel is a vampire: he should not cast any reflection.
Revealing mistake: When Groo rushes in to battle the tree demon, a cobweb gets stuck to the camera as it pans in.
Deliberate mistake: In the flashback to Angelus rising from his grave, his and Darla's breath is visible as they exhale. This was going to be removed in post-production, but left in due to budget restrictions.
Revealing mistake: Supposedly the demon Merl has no tongue, but when he is telling Angel the apology he wants, it can be seen. (00:01:00)
Other mistake: In the episode "The Shroud Of Rahmon," when Angel changes into his vampire face, his teeth don't change into fangs. (00:33:55)
Continuity mistake: In the season 5 episode 'Destiny' we see that the first time Angelus meets William, he has already adopted a more cockney accent, but in an earlier episode of BtVS (Fool For Love) Angelus asks Spike "Since when do you talk like that", suggesting he changed his accent after they met.
Continuity mistake: The entire area around Cordelia and Skip is supposed to be frozen in time. However, there is a car driving freely across a bridge in the background behind Cordelia. (The background is blurred, so it is difficult to see.)
Disharmony - S2-E17
Continuity mistake: In 'Disharmony' Willow's hair is over her eye when she says "Thanks for the affirmation", but when we see her again a second later its tucked behind her ear. The shot wasn't on Cordelia long enough for her to have tucked it back. (00:16:45)
Character mistake: When the bidding for Cordelia in the auction reaches $20,000, somebody bids for $30,000 after the auctioneer has said "going once, going twice" and they end up winning. The auctioneer should have in fact restarted saying "going once, going twice" after the $30,000 bid was made.
Quickening - S3-E8
Continuity mistake: Sahjhan establishes that he is incorporeal on Earth, but when he takes Holtz to meet his demon minions, he manages to knock on the door.
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.
A New World - S3-E20
Revealing mistake: The Axe that Groo throws at Conner at the start of the episode (Obviously CGI) doesn't match the movements of his hand, it leaves his hand before he actually throws it. (00:01:40)
Revealing mistake: During the whole slow motion fighting scene in the bar the punches clearly miss their targets.
A New World - S3-E20
Revealing mistake: The first stake that Conner fires from his stake cannon (Obviously CGI) doesn't seem to emerge properly from the barrel of the cannon, it emerges out of thin air from the side. (00:01:25)
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.