Dr. Laing's balcony has open air above it as it is protruding from the balconies of higher storeys. He lives on the 25th floor, but from the exterior pictures of the high rise you can see that only the highest 10 have balconies like that, so those would only start at the 30th floor.
When Royal descends to the lower floors, he is seen with and without his blood stained jacket.
When Laing cuts into the human head during the pathology / dissection scene, blood is shown flowing from the fresh incision. This is medically impossible, as blood ceases to flow once a person is deceased; even more so when the head has been long since detached from the rest of the body.
At one point there's a car right outside the building with a clearly made-up number plate. Something like DAE 080X. The leading zero is incorrect, X is 1981 (the film is set in the 1970s) and the number plate font is the newer post-2001 one.
When Laing first meets Royal, trees from the garden location which should have been digitally removed can be seen behind the wall in one shot. There should only be a sheer drop as it's meant to be a roof garden at the top of a tower block.