Other mistake: The vice principal's mispronunciation of Carrie's name (immediately corrected by Miss Desjardin) is given as "Cassie" in the DVD subtitles, but as "Casey" in the audio. (00:11:07)
Other mistake: After Carrie levitates herself and goes outside, there's a wet spot on the concrete that's the size of both her feet, showing her landing spot from previous takes.
Other mistake: The alternate director's cut ending available for viewing within the movie via seamless branching on the Blu Ray is very lazily cut into the film. The music from the previous scene cuts out abruptly when the scene begins, Sue's stunt double's face appears in a close-up shot, and the music from the ending credits abruptly cuts in partway through the song (rather than starting the song from the beginning) once the alternate ending is finished playing. The very end of Carrie's scream from the theatrical ending can also very briefly be heard at the same time the music begins.
Answer: Sue just woke up from a terrifying nightmare in which Carrie's arm reaches out of her grave and grabs Sue. A lot of hospitals have crosses in the rooms. It's been suggested that Sue might have miscarried and lost her baby.