Early in the movie, when Marni first finds out Joanna will be staying in her room, you can clearly see two twin beds. Later, when Will comes in to give Joanna the rose, there is only one large bed. The big bed is seen multiple times throughout the movie.
When Tim tries to give Marnie the umbrella drink, as she turns to look at Tim the umbrella nearly pokes Marnie in the eye. In the very next scene the umbrella is at the back of the glass and nowhere near Marnies face.
Just after Gail drop the earring down the drain and kicks off the waste pipe to try and retrieve it causing her to get soaked, she puts the waste pipe back in place and the water immediately stops pouring even though a loose pipe is clearly usable and the waste pipe has nothing to do with water flowing to the sink.
After Joanna presents Marni with the MOO necklace, it disappears from around her neck, before reappearing shortly afterward.
At the beginning of the film, Marni is a Vice President at the Los Angeles office a large PR firm and has just been promoted to a position in New York. Since Marni was a High School sophomore eight years earlier, this would mean she would have graduated from college and started working full-time about two years earlier. Rising to VP in a large, established firm that quickly isn't possible.
When Marni is helping grandma Bunny with her dentures, you can clearly see that grandma Bunny has all her teeth.
When driving past Richfield High, James Wolk as Will looks directly in the camera lens after Odette Annable as Joanna says "she did not!"
When Mona and Gail fall into the pool, the underwater shot and the way they are moving once they resurface is 6 feet deep from the sign on the side of the pool. However, the next shot shows the sign on the side of the pool which says 4 1/2 feet, and they are still treading water as if to indicate they're in over their heads. 4 1/2 feet though is shallow enough that the characters could stand on the bottom while having their conversation.
On Marni's first night home, while eating dinner, Marni says "oh dad" while putting a fork in her mouth with no food. She is putting to chew on food.
When Ramona pushes Gail into the pool at their senior prom in 1976, "Barracuda" by Heart can be heard. The song had not been released yet as it first appeared as a single in January 1977. It is acknowledged by the characters later on, so it cannot be "just the soundtrack".
From the look of their cheerleader uniforms, Bunny and Helen appeared to be in high school in the 1930's or 40's. "Who's Sorry Now" by Connie Francis wasn't recorded until 1958.