Carpe Noctem - S3-E4
Continuity mistake: When the gang rescues Angel and returns him to his normal form towards the end of the episode, Cordelia shocks him unconscious with a taser and he falls to the floor, lying on his front. But when Fred whacks him on the head with her baseball bat, he is suddenly lying on his back.
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.