Revealing mistake: When Angel stands at the cabinet with a file folder in his hand, you don't see the folder in the reflection in the window (which Cordelia uses as mirror to put on her lipstick) but you can see the cabinet. (00:00:20)
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)
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.