When Hightower visits Gardner in his office he is addressed wrongly as 'Lieutenant' multiple times although his Captain's bars are clearly visible and he was addressed with his correct rank in their first encounter earlier in the movie.
The tape on Gardner's glasses changes.
The blood on Gardner's face/shirt changes.
As usual in these movies that show vintage cars, they are absolutely immaculate, shiny, and squeaky clean because they've all come out of auto museums. You never see a car with a dent or one that's not highly waxed and polished.
The Tonka truck on the Mayers porch was a model not manufactured by Cat Trucks until 1975.
In the introduction, Suburbicon is described as having a shopping mall. In the 1950s, they were called "shopping centers." The use of the British term "mall" in this sense did not begin to take hold in America until the mid-60s.
The term Mall is only really used in North America and is not a British term. Malls are called shopping centres or shopping precincts in Britain.