Revealing mistake: When Groo rushes in to battle the tree demon, a cobweb gets stuck to the camera as it pans in.
Continuity mistake: When the demon Angel and Groo are chasing crashes through a wall, the resulting hole lets in enough sunlight to force Angel into the shadows. A few shots later, Wesley notices Angel standing in the entrance to the hole. Only a few moments have passed, so Angel shouldn't be able to stand where he is. (00:15:20 - 00:16:20)
In The Dark - S1-E3
[Watching from on top of a building as Angel talks to a woman he just saved, Spike guesses what is being said.]
Spike: [as Rachel.] How can I thank you, you mysterious black-clad hunk of a night thing?
[as Angel.] No need, little lady, your tears of gratitude are enough for me. You see, I was once a badass vampire, but love and a pesky curse defanged me. Now I'm just a big, fluffy puppy with bad teeth. [Rachel reaches for Angel's head.] No, not the hair. Never the hair.
[as Rachel.] But there must be some way I can show my appreciation?
[as Angel.] No, helping those in need's my job, and working up a load of sexual tension and prancing away like a magnificent poof is truly thanks enough.
[as Rachel.] I understand. I have a nephew who is gay, so...
[as Angel.] Say no more. Evil's still afoot. And I'm almost out of that nancy-boy hair-gel I like so much. Quickly, to the Angel-mobile, away.
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.