A New World - S3-E20
Revealing mistake: The first stake that Conner fires from his stake cannon (Obviously CGI) doesn't seem to emerge properly from the barrel of the cannon, it emerges out of thin air from the side. (00:01:25)
A New World - S3-E20
Revealing mistake: The Axe that Groo throws at Conner at the start of the episode (Obviously CGI) doesn't match the movements of his hand, it leaves his hand before he actually throws it. (00:01:40)
A New World - S3-E20
Revealing mistake: When Connor runs out of the hotel and Groo and Gunn drag Angel out of the sunlight, Angel's reflection can be seen in the door behind him just as Cordelia steps outside. Angel is a vampire: he should not cast any reflection.
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.