At 1:12:20, Will is walking quickly to the building for the pageant opening. Millie yells to her "You're LATE". Willowdean replies, "Well, I told Lee I needed my wardrobe by noon and he said 'I'm a designer, sugarplum, not a magician'". This indicates it's past noon. But about 45 seconds later, when Willowdean's mom walks onstage, she says "Hello, good morning, girls." Past noon is no longer morning.
When Rosie is telling Willowdean the story of Lucy's helping her with the pageant dress; the orientation of Rosie's arms changes from one cut to the next.
Most of the cars, including Rosie's and Willowdean's, do not have front license plates. Currently the state of Texas requires all motor vehicles to have both front and back plates. However during a period between 2013 and 2015 it was allowed to wear only the back plate. This period was the time when the novel was written and published and where the filmmakers decided to set the film.
At about 44:40, when Will and Elle are talking to each other on their cells, it's obvious from the displays that the phones aren't actually in use.
When Willowdean is ready Lucy's pageant application at 24:54, Lucy's age and birthdate are visible. The birthdate shown is 5/5/79. which is incorrect; it should've been 5/5/77, assuming that Lucy was 16 years old in 1993.
When Ellen runs upstairs to get Will for the first day of school (18:27), she wears a jean-skirt and halter top; later in school she wears denim overalls and a T-shirt. She had enough time to change herself, but not enough to wait for Will to change.
Lucy's pageant application is shown. It states the date she fills out the application as "August, 1993". Further down it says her age is "16". Next to that is says her date of birth is "5/5/79". That date of birth would make her 14 at the time of the application, not 16.
When Will finds the flyer for the "Dolly Parton Night Every Saturday Night" in her aunt's scrapbook, it has no address on it anywhere to state where the The Hideaway is. How did Will and her friends know how to find the place, then claim at the door to not know it was a bar?
At the finale when they announce that the show is sold-out; Millie's mom discovers she is entered into the pageant; she decides to stay and somehow manages to get a seat.