Continuity mistake: When Mia arrives at the cafe she works at the morning after her and Sebastian first dance together, she has a yellow bag, but when her and Sebastian walk out of the cafe later, she has a larger red bag.
Audio problem: Seb and Mia are watching "Rebel Without a Cause", and the scene shows Jim Stark driving up the ramp to the observatory. The music playing at that moment is the original track from that movie, but it is not synchronized properly with the visuals. (00:51:06)
Visible crew/equipment: In the opening sequence when the crowd dances in front of the band inside the truck on the highway, you can see the shadow of the camera on a man's back as it makes an aerial shot.
Other mistake: During the opening traffic jam dance scene, many of the cars have dented roofs and hoods from previous takes.
Continuity mistake: The first time Mia meets Sebastian, during the traffic jam, she raises her middle finger then quickly lowers it. When she meets him at the bar and we see the scene again, she raises her finger, then waves the hand in denial and, finally, lowers it.
Continuity mistake: 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. (00:03:45)
Visible crew/equipment: When Sebastian and Mia go to the planetarium, the camera crane is reflected on the whole right side of Sebastian's car.
Continuity mistake: After Mia and Seb are walking through the backlot, and he asks her what she's doing right now, when she says "nothing" the image is flipped, her hair is reversed and her bag is on the opposite shoulder.
Continuity mistake: After Mia's firefighter joke, when Sebastian says "So you're an actress" she's leaning sideways, but in the next shot when he says "I though you look familiar" she's facing him. (00:29:52)
Other mistake: When we are shown Mia watching a YouTube clip with an interview with The Messengers, the clip is actually shown as paused but is still playing. (01:08:40)
Continuity mistake: When Sebastian is talking with his sister in his apartment, the clock on the wall strikes 2:55, in the next shot it strikes 10:50.
Continuity mistake: After Sebastian's sister leaves his apartment right after she's been sitting on the stool and writing something at the table, he starts practising at the piano, but the table is missing and the stool is in the corner.
Chosen answer: The answer is, "It's a movie." They wouldn't have been able to drive up to the entrance either. The entire sequence is fantasy-driven.