Revealing mistake: You see Cordelia and Wesley walking without Angel through the gym in the mirror (because vampires have no reflection). In the Widescreen Edition you can see Angel waiting at the right side of the screen to join in a "normal" shot. (00:02:45)
Revealing mistake: Just after Angel kills the quan-eyed demon, he exits the room behind the smashed mirror. On the widescreen DVD you see Angel's reflection in the mirror to the right of the smashed mirror (even though vampires don't have a reflection). (00:03:40)
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 Wesley accidentally turns off the power, the room briefly goes dark, then lights up with moonlight/streetlight from outside. In the real world, that light would have been visible the whole time in the background. (00:01:45)
Answer: Everybody including Angel saw her. After helping them and then being left alone to talk to Angel, Cordelia says that the Powers That Be let her come back one more time to help them. After Angel gets a call saying that she died, he realises that it was her spirit that was helping them the entire time.