Other mistake: During the opening traffic jam dance scene, many of the cars have dented roofs and hoods from previous takes.
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: 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.
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.