The Cautionary Tale Of Numero Cinco - S5-E6
Revealing mistake: When Angel says to Wesley that "Fred has got something," and then walks out of the room, Angel's reflection is visible in a window pane in front of him. It's very brief, but it's there. (00:31:05)
The Cautionary Tale Of Numero Cinco - S5-E6
Factual error: When the mysterious person hands Cinco the business card during the 50's flashback (in the restaurant where he is sitting waiting for business), it says "Wolfram & Hart, Attorney's at Law." Clearly, there should be no apostrophe in "Attorneys" in this context - not a mistake a professional law firm would make.
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.