Early in the film, Susan finds photo negatives in the trash can. When she holds them up to the light and they show her face, she is clearly not wearing makeup.
When her husband Jack walks in seconds later, they show Susan's face and she is wearing dark orange lipstick. Seconds later, they show Susan's face again and she is wearing no makeup.
When Jack is sitting in his bedroom eating Chinese food, he offers a shrimp to his wife Susan. She eats it off the chopstick, starts to chew it. But in the next moment she speaks and her mouth is empty, yet she didn't seem to have swallowed.
Susan's weight fluctuates noticeably from one scene to the next. Since the entire film takes place within a couple of weeks at most, this is unlikely.
Susan's hair color changes from light to dark & back again as the movie progresses. It also goes from shorter to longer & back again. Since the entire film takes place in a few weeks at most, this is unlikely.
We both see and hear Jack's friend and colleague Steve several times, even at the end of the film Susan clearly calls him Steve. But when Susan is coming home from dinner at her mother's house, the phone rings. When she answers it, we clearly hear Steve's voice. But Susan calls him Larry.