Revealing mistake: In the last scene of the episode, Angel kicks the guy out of the glass building. This guy's a vampire, and you can see his reflection in the glass windows. Vampires aren't supposed to have reflections.
Revealing mistake: In the scene when Angel, a vampire, runs after the barman towards the end, after the barman has tried to pass on the demon to Kate and Angel stopped him, there is a mirror, and when Angel runs past it, he has a reflection. (00:38:20)
Visible crew/equipment: In the scene where Angel is talking to Kate about the "evil thing" , the boom mike appears at the top of the frame. (Widescreen Edition).
Continuity mistake: In the season one episode 'I Will Remember You' when Buffy and Angel are talking in the sewer Buffy's hair goes from being just a few strands over her shoulder to having a whole side over it, and back again. (00:11:05)
Revealing mistake: In "To Shanshu in LA" in season one, as Angel is pulling out of parking lot to follow the W&H Trucks, you can see his reflection in his rear view mirror (vampires have no reflections). Also odd that throughout the first season the rear view mirror is removed in some episodes. (00:31:19)
Continuity mistake: In Season One Finale, "To Shanshu In LA" when Wes is lecturing Cordy on wants and desires, the number of bites taken from the doughnut changes, in one instance it breaks in half, most visibly when Wes takes a bite from Cordy's doughnut on the angle change. (00:13:31)
Visible crew/equipment: At the end of the episode where Wesley is studying the Shanshu scroll in Cordillia's apartment, the boom mike appears at the top of the frame TWICE.
Continuity mistake: In the season one finale "To Shanshu in LA," near the end Angel is fighting Vocah in the mausoleum and Angel rips off Vocah's mask, revealing the maggoty face Underneath. In subsequent shots of Angel from behind Vocah, you can still see the edges of the mask on his face. (00:34:30)
Continuity mistake: In the final fight, Angel gets stabbed on his right side. In the slow-motion bit before his opponent chooses not to fight any more, the wound switches to his left for one shot. (00:36:40)
Continuity mistake: When Wesley is explaining that Angel feels like he doesn't belong in the world, the amount of donut he's holding changes. (00:13:30)
Visible crew/equipment: When Angel is fighting the demon a cameraman is visible rising on the lower right. (00:27:13)
Visible crew/equipment: When Angel kneels down to the fridge a camera lens is visible in the top left. (00:15:21)
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.