Angel (1999)

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.

Shay

The House Always Wins - S4-E3

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)

Shiny Happy People - S4-E18

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.

THGhost

Salvage - S4-E13

Revealing mistake: When Wesley and Faith jump out of the prison window and land on a car, the face of Wesley's stunt double is visible as they land.

THGhost

Release - S4-E14

Revealing mistake: After Wesley stabs the junkie at the demon bar and finds out where Angelus is, he wipes his knife clean. But the knife is already clean before he wipes it: there's no blood on it at all.

THGhost