Sleep Tight - S3-E16
Revealing mistake: When Angel is seen gulping the blood from the cup (right before Lorne tells Angel that Connor needs some papa-love) you can see this is a special-effects cup (it's like one of those freezer mugs, where there's a layer of liquid on the outside). If Angel were really drinking out of the cup the way it's shown, he would have blood all over his lips and it would be running down his chin.
Visible crew/equipment: When Angel is fighting the demon, you can see an obvious camera and two hands in the bottom right corner. (00:27:10)
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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.