Continuity mistake: When Angel kicks the vampire out the window, he does it with little effort and movement, but when the shot goes from outside the building, he's swinging his body like he's put more into it.
Continuity mistake: In the scene where Detective Lockely arrests Angel in the apartment, the window behind them shows that it's daytime. But when Angel jumps out the window, it's night time.
Continuity mistake: In the season one episode 'I Will Remember You' when Buffy and Angel are talking in the sewer Buffy's hair goes from being just a few strands over her shoulder to having a whole side over it, and back again. (00:11:05)
Continuity mistake: When Angel and Buffy are sitting down talking about their kiss, Buffy picks up a coffee mug and holds it with both her hands. But in a wide shot shortly afterwards, she is no longer holding the mug and it has moved further down the table.
Continuity mistake: When Cordelia is putting on lipstick she uses the windows as mirror. In the reflection we see the top drawer of the cabinet is open the whole time, but it is later opened by Angel. (00:00:10)
Continuity mistake: In the final fight, Angel gets stabbed on his right side. In the slow-motion bit before his opponent chooses not to fight any more, the wound switches to his left for one shot. (00:36:40)
Continuity mistake: When Faith is sitting at the window and looks at the piece of broken glass she used to torture Wesley, it has a different shape than the one she first takes to torture Wesley. When she drops it out the window it has its original shape again. (00:33:10)
Continuity mistake: In the rooftop scene, Buffy's hair is partly tied back and slightly curly but by the time she reaches the Police station it is completely loose and much curlier.
Continuity mistake: When Wesley is explaining that Angel feels like he doesn't belong in the world, the amount of donut he's holding changes. (00:13:30)
Continuity mistake: In Season One Finale, "To Shanshu In LA" when Wes is lecturing Cordy on wants and desires, the number of bites taken from the doughnut changes, in one instance it breaks in half, most visibly when Wes takes a bite from Cordy's doughnut on the angle change. (00:13:31)
Continuity mistake: In the season one finale "To Shanshu in LA," near the end Angel is fighting Vocah in the mausoleum and Angel rips off Vocah's mask, revealing the maggoty face Underneath. In subsequent shots of Angel from behind Vocah, you can still see the edges of the mask on his face. (00:34:30)
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.