Angel burns his right arm to fetch the key in the holy water and uses the same hand to unlock the door. However, the close-up shows him unlocking the door with his left hand.
Drusilla draws her razor sharp fingernail across her chest for Darla to drink. The bloody cut on Drusilla's chest already exists as Drusilla draws her finger below the cut.
When Angel drags the creatures' upper half away from the lower in the first trial, you can see the green suit the actor is wearing to make the lower body invisible.
When Angel bisects the demon during the trial, the stunt harness used can be clearly seen.
In the episode where Darla, and Angelus are chased by a vampire hunter in France, in 1765, Angelus tells Darla that he hates France and they should go to Romania instead. In 1765 Romania did not exists, it appeared as a state in 1859 as a result of the unification of two Romanian provinces: Moldavia and Valachia.
When Cordelia says the fine line, "Don't you think we should check on him?" her lips don't move.