Heartthrob - S3-E1
Revealing mistake: When James, a vampire, chases Angel and Cordelia into the subway, James' reflection is visible in the windows of a passing train.
Heartthrob - S3-E1
Revealing mistake: In this episode both Angel's and James' reflections are visible during the fight scene inside the train.
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.