Other mistake: Both times we see Lindsey shaving, once before his hand transplant and once after, his reflection in the bathroom mirror isn't in fact a reflection. He is wearing a bracelet on his left wrist, but in the mirror it is on his right wrist.
Other mistake: Later in the day when Cordy has recovered a little and is cleaning, Angel says something about seeing a reflection in the picture. We hear Cordy say, "What do you want," but the subtitles say, "What do you what?"
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.