The day Sebastian visits Mia at her coffee shop, Mia entered work wearing black pants with a yellow bag. When Mia leaves work, she is wearing a blue skirt with a red bag. When they show a long shot of Sebastian and Mia walking on the lot, she is seen to be wearing black pants. Then in the next shot, she is wearing the skirt again.
During the road rage incident between Mia and Sebastian, Sebastian is seen wearing a brown-colored shirt. When he returns to his apartment and has the conversation with his sister, he is wearing a blue-colored shirt. In the next scene, when Sebastian is practicing on his piano, he is back to wearing the brown-colored shirt. Director Damien Chazelle acknowledged this mistake in the audio commentary.
In the first scene, the dancing performance is not continuous based on the shadow pattern. When the back door of the truck is opened and the music band starts playing, people dance behind the truck. Then the camera turns 180 degrees. From here, the sun's shadow pattern is changed which shows that the sun has moved in the sky and the scene is not continuous.
Throughout the movie, Mia has her hair parted on the right side. This includes of course the scene where she walks outside the studio buildings with Sebastian. However right before the end of the walk, there is a close shot of her where her hair is parted on the left side. In addition the background of the close shot features a black and white building or trailer which wasn't there during the walk.
Before the end of the 'Another Day of Sun' sequence, as the camera pans up over the freeway, we don't see Mia or Sebastian's cars. After the 'La La Land' and 'Winter' title cards and the camera pans back down, their cars suddenly appear, replacing the other cars that were there before. Mia's Prius replaces the white SUV with the dancer in orange on top, while Seb's car replaces the gray sedan behind the SUV with the dancer in green.
When Mia gets the phone alert about her audition in the first coffee shop scene, the date is January 25 - but subsequent scenes take place before Christmas, including the pivotal scene where Seb is supposed to be playing Christmas carols and gets fired.
At the very beginning of the movie, when Mia is leaving work to go to the audition, she bumps into a customer who spills a cup of coffee on her white shirt. You can clearly see that when she leaves the audition, the big coffee stain on her shirt is very different from before (when she was leaving work).
Right before they meet Keith for the first time, as they are sitting down at the table, something (necklace?) falls off of Mia onto the table, Sebastian quickly swipes it away with his right arm as he takes a drink with his left. When the camera switches, he is drinking with his right arm.
At the beginning of the movie, Mia is wearing a woman's blouse (buttons on left) when she leaves work for an audition and bumps into a customer who spills coffee on her. During the audition she is still wearing a woman's blouse, but when she leaves you can see that she is wearing a man's shirt (buttons on right) as she unzips her jacket and enters the elevator. When she gets to her apartment and flops on the bed, she is back to wearing a woman's blouse.
In the Lighthouse cafe, before Sebastian met Keith for the first time. Sebastian took Mia's hand and went across the dance floor, sitting by the table of two. Mia went too fast and hit the table slightly while something, possibly a recording device or Mia's left earring, fell on the table and Sebastian caught it by hand. Then he smoothly collected it in his pocket without explaining what it was or returning it back to Mia.
During the opening freeway dance scene, several of the cars used for dancing have two visible, color matched strips on both the hood and roof. These are textured anti slip adhesive to prevent the dancers from slipping.
When Sebastian is playing the opening piano solo for "Start a Fire" and he looks out into the crowd to smile at Mia, the people around Mia are already moving their heads to the main rhythm of the song (which hasn't kicked in yet)
When Mia checks her phone for her first audition in the opening scene, the date reads Monday, January 25th, the day that they filmed the scene. Howeve it is insinuated that it's Christmastime, and therefore her phone should read a date in December.
When Sebastian goes to check on the oven after the argument with Mia, the smoke rises out of the oven slowly and almost horizontally, a sign that fake smoke was placed inside. If the oven had actually started smoking due to extreme heat overcooking the food, the smoke would rise vertically and extremely fast upon being opened.
At the beginning of the movie, after the crowd dancing scene, the camera quickly pans to the left to the girl in the yellow dress lying on top of the red car. After the pan, all the shadows of the people are pointing in the opposite direction, revealing that they shot the latter scene at a different time of day and the shots were combined to make it look like a single continuous shot (at 3 min 45 sec).
When Mia and roommates are getting ready to go out, one of her roommates pulls out a strappy blue dress and gives to Mia to wear. Mia dances and sings with the dress insinuating that she is going to wear it out that evening.
When the girls all are finished getting ready and head out Mia is seen wearing a completely different blue dress that looks nothing like the one she was handed by her roommate.
It is same color, but different style of dress altogether.