When Hindly returns from flogging Heathcliff, everyone is seated with drinks in their hands preparing for a meal. At a different camera angle everyone has a plate on their laps.
Hymn anachronisms are very common in many British TV historical dramas. In this film, a church congregation sing the hymn 'Holy, Holy, Holy' by Reginald Heber, but use the tune 'Nicaea' by J. B. Dykes, which would not be written until 1861; over 30 years after Cathy's death in 1830 (as listed on her tombstone in episode one).
The drive leading to the Manor has grass in the middle, a modern phenomena. In the 1840's the drive would be churned up and bare as a result of horses pulling wagons. Also numerous modern tracks visible.
Numerous covers visible over modern light switches particularly in episode two when Edgar confronts Heathcliffe at the Manor.
When Nelly meets Heathcliff on the road there are some modern-looking tyre marks visible in the mud.