Continuity mistake: When Ana and Christian are having sex on the dining room table, she drops Christian's pants below his butt. Scene cuts and Christian's back is shown, with his pants above his butt. Then cuts back and they're below again. (00:54:40 - 00:56:50)
Continuity mistake: When Jack calls Ana from Mia's phone, he makes his demands and throws Mia's phone in the water. Then from the bank Ana calls Jack on a cell phone.
Continuity mistake: When Ana comes home and Christian is coming down the stairs with a blindfold, the staircase is just a handrail, whereas throughout the rest of the movie, the staircase is enclosed with a white wall.
Continuity mistake: When Anna is lying on the concrete and you hear Christian say, "Anna, breathe," she disappears while cops are arresting Jack.
Answer: The movie's timeline is not clear, but in the book, it happened during the honeymoon, way before the next Depo shot was due, so it's not because she forgot or was negligent of it. The shot ran out too early, which is rare, but it does happen.