Cops stop the psychedelic hippie hearse right in front of the same supermarket that is seen in background several miles back when they began pursuing vehicle in the opposite direction.
In the beginning of the movie, Peter Sellers is driving his black Lincoln Continental. When he stops for a traffic light, a cream-colored 4-door 66 Chevy Impala or Caprice with a black vinyl top pulls up behind and one lane to his right. The view switches to Sellers viewed through the windshield, and the car has changed to a 2-door cream-colored 64-65 Chevelle convertible with a white top.
When Harold and Nancy first make love, the light shining on Nancy's butterfly tattoo makes it clear that it is a decal, not a real tattoo.
While looking for a parking spot in the garage, the first time Peter Sellers comes down the ramp to level B, the first spot next to the ramp has no car parked in it, yet Sellers continues to look for a spot.
When Nancy (Leigh Taylor-Young) opens a cupboard in Harold Fine's (Peter Sellers) apartment, there's a shelf containing four jars of Safeway Instant Coffee. The jars are packed tight with other items on the shelf. When Howard gets another jar of the same coffee from his mother, he opens the cupboard and places it next to the other jars of coffee where there was no room earlier.
At Harold and Joyce's wedding, the first words spoken by the Rabbi do not match the video.
In the second scene in the film, when Harold is driving his car, the shadow of the camera with a cable coming out of the side of it is visible on his arm.