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)
Revealing mistake: In the scene where Angel fights the demon for the first of his trials, upon cutting the demon in half for the second time he chains the two halves to the walls. When he drags the upper half of the demon to the wall, the green legs of the actor are momentarily visible, when they should have been removed by CGI. (00:29:00)
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.
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.