Deliberate "mistake": When George spies on his hedonistic neighbor down the hill with his telescope, the telescope is always level when it actually should have been pointing downward. Same when his neighbor spied on him - his telescope was always level also when it should have pointing upward.

Visible crew/equipment: In the film George is backing his Rolls Royce up at Hugh's place, as he's doing that the film crew's reflection can be seen on the car.