When R and Julie hit the car in the parking garage, the BMW has a dent in the grille. When they pull up in front of the abandoned house, the dent in the grille is gone.
After it has the accident in the parking garage, the red BMW convertible appears with clearly less damage in later scenes.
Either the scenes were filmed out of order, or they neglected to replicate the damage on the duplicates.
The same Buick Riviera appears in multiple locations about town, including the industrial area through which the Corpses walk and outside the pharmacy the Living raid.
When Julie runs out of petrol on her way back to the fortified city, her BMW rolls to a halt next to a derelict vehicle. In the following aerial shot the BMW is standing on a free lane with the nearest other car several metres away.
When Julie takes a picture of R with the Polaroid camera, she has it on an angle but the picture shows R perfectly straight.
When Julie parks the BMW convertible in the suburbs to get out of the rain, the ratchet sound of a mechanical hand break is audible. A BMW Z4, type E89, has a electrically operated hand break which creates a whirring sound.
When Julie and R are running away from the Bonies, they run through the parking garage to use the red convertible to get away. As Julie is getting into the car, she grabs the windshield to sling herself in the driver's seat. Since the windshield is dusty, you can see where she grabbed the windshield from past takes of filming.
As R and Julie talk on a highway, behind R, there are two jet contrails visible in the sky.
When R sneaks in to the city from the hidden tunnel he leaves the exit open yet later when he and Julie go to back to enter it they had to remove the makeshift door to get in.
When R realizes that he can freeze he's walking in a downpour. Due to the chosen perspective, you can see the rain end a few meters behind him, revealing that it's coming from a hose on a crane rather than being real rain.
After Julie abandons R and he walks away from the house, one shot shows contrails from aircraft in the sky above him.
When Julie goes out to the balcony, she leaves the door open a crack. But as Nora interrupts Julie on the balcony, the door makes a distinct unlatching sound.
When we hear "Shelter from the Storm" by Bob Dylan, we see an LP being played. If we assume this LP is Blood on the Tracks, the needle placement is incorrect. Shelter from the Storm is the penultimate track on Side 2, meaning the needle would be more than halfway toward the center label. What we in fact see is the needle not yet halfway through the side. This would indicate we should be hearing "Lily, Rosemary and the Jack of Hearts."
To decide to call R by the name "R" is only between Julie and him, but still his zombie friend (later revealed to be) Marcus calls him that when they meet on his way back to the airport.