In the ICU scene after the bonfire, the shot of the bed with the rails down, when they change perspective to Mia's point of view, the right bed rail is up, then when the scene goes back to the overall room shot, the rails are back down.
When the car crashes there is snow on the trees and ground, but when they are brought to the hospital and Mia looks out the window, there is no snow and it looks like spring outside.
At several points in the film, other actors step out of the way so Mia can pass by them for the purposes of the scene, even though she is invisible and intangible.
Mia wanders barefoot around the hospital halls as a ghost, but in some angles, the soles of her feet are darkened and soiled, most probably by the dust/dirt of the hospital floor.
Mia's letter from Juilliard gives the place and time of her live audition, but no date for the audition.
At the Labor Day party when Mia is talking about the best day of her life, she and Adam jam together while playing the guitar and the cello. When the scene pans out and Mia is narrating, the girl playing the cello is not Chloë Grace Moretz, it is some other girl.