When Frank drives his van back to his studio apartment, it's night. Upon changing scenes, it's now day - not sunrise in this case as it would take quite some time before the sun would come up.
When Lucie meets Frank at the restaurant, her underarms seem freshly waxed. Later in her apartment, there is visible gray stipple under her arms. This is likely a continuity error because hair does not grow that fast and the scene in question takes place immediately after the restaurant.
The song Lucie puts on in her apartment never skips despite multiple jump-cuts.
Despite the film taking place from Frank's perspective, the shot where he follows Rita to her home reveals his shadow to the right - not taking place from Frank's POV.
When Frank is being monitored in a convenient store window, he grabs the blinds but his right hand clearly shows it in a different position from the security monitors.
When Lucie puts on music at her apartment the record is playing the 7" version of Q. Lazzarus's song "Goodbye Horses" while the actual record is clearly a 12".
(around 1 hr. and 12 min.) Shadows pass over behind Anna, just before Frank blocks the front door.
When Frank has a panic attack and rushes off to the bathroom to take his meds, he gapes at his reflection and the camera's extension cable can be seen drifting into the shot, several times.
As Frank smashes his lamp on his girlfriend mannequin, his shadow reveals the camera (prominently its extension cable) used to film the scene.
Frank's reflection on a passing subway vaguely reveals the cameraman and camera assistant.