Several fonts used did not exist in the 1960s. The university office door signs and the movie's titles use Gotham, introduced in 2000. The bank logo uses Trajan, created in 1989, and Gill Sans, popularized in the US in the 1970s.
The aria "Ebben Ne andro lontana" from Catalani's opera La Wally is used in the film. Naxos released the recording in 1992. Singer Miriam Gauci was born in 1957, which would make her 5 years old at the time of the movie.
When George is leaving for work in his car, and looking at the girl who lives in the house across the street dancing in slow-motion, the house behind her has dimensional-style shingles, which were first available in the 1980s.
When George peeks at his neighbors from the toilet, his neighbors' house is on the right side of his house. When he leaves for work, he drives from the right side of his neighbors' house, which suggests his neighbors are on left side of his house.
The goof item below may give away important plot points.
In the final scene, when George's heart attack starts, he sits on the bed, lays his left hand on the right side of his chest, then grabs his right arm. While the pain is usually on the victim's left, it can occur on the right. In subsequent shots, when George has fallen to the floor, he holds his left arm with his right hand.