Ground State - S4-E2
Revealing mistake: In 'Ground State' when Gwen is walking towards the lift in Irwin's building you can see Angel's reflection on the shiny door. Vampires don't have reflections, as is demonstrated at various other times in this show.
Revealing mistake: When Gunn, Fred and Lorne run out to the Las Vegas strip, a large crowd of bystanders can be see seen in the background watching the show being filmed (as said in the DVD commentary). (00:28:25)
Revealing mistake: When the gang attack the vampires in the bowling alley, watch as Jasmine and Fred walk over the bowling lanes in the background as the others fight the vampires. Jasmine slips on the floor but manages to maintain her balance. You'd think a deity with superhuman strength, telepathy, shape-shifting and psychic capabilities would be able to walk on a smooth floor without slipping and also know that it was smooth to begin with. Although she may lack friction, the shot should have been re-filmed as it is a blatant mistake.
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.