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)
Continuity mistake: Right after Cordelia has her vision, Wesley goes to the whiteboard. In the "Case" column he writes "N.D.U.O." Underneath what looks like "CARYNSS" crossed out. But once they find out that it is a Prio Motu demon and Angel writes it on the whiteboard, "N.D.U.O." disappears even though Angel did not rub it off.
Continuity mistake: When Angel writes "Prio Motu" on the whiteboard and says that they need to find it and narrow it down to somewhere, Wesley says "I may be able to help us with that." In the immediate closeup of Angel, his stance has suddenly changed. In the previous shot he was standing with his arms straight down and his hands close together, but in the closeup they appear to be as though he has his hands on his hips.
Visible crew/equipment: When Angel is fighting the demon who protects the pregnant woman, at one point a crew member (in ordinary street clothes) shows behind them for a few frames.
Visible crew/equipment: When Angel gets on the horse to joust, you can see a part of a camera at the right side of the screen. (Widescreen Edition)
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.