When Richard is reading the statistics in the doctor's manuscript, a stat is given that the seven year itch affects 84% of men in their 7th year of marriage. The next stat says that this number increases to 92% in the summertime. However, this makes no sense. If the stat is that 84% of men in their 7th year of marriage get the itch, then how can it change depending on the season? The logic does not follow.
When Richard is talking on the phone, he moves to the porch during his phone call. At first, the cord of the phone is seen coming around the corner through the doorway, but later disappears and eventually reappears.
In the scene where Richard opens the door for The Girl, at the time she forgot her front door keys, he gets out of the apartment to talk to her and look at her going upstairs. When he gets out the lights are on, but when he come back after he hurt his neck, the lights are off.
The stairs in Sherman's apartment are explained by the fact that his and The Girl's apartment used to be a single dwelling that was divided up. However, there is already a staircase in the hallway that leads to The Girl's apartment, and the positioning of the two staircases means they would in fact meet at the same spot on the upper floor, which is a very unlikely architectural design.
When Helen shoots Richard in his fantasy, seven shots are heard although the gun is a six-shot revolver. Moreover, she enters the apartment by emptying the gun into the front door, yet she doesn't reload. Some of the bullet holes disappear between shots. Since this is Richard's fantasy, it need not conform to reality.