I recently read the Buzzfeed article about S01E05 of Game of Thrones and one of the bullets on the list said:
Cersei mentioning losing their first boy to Robert confirms that she wasn’t making up that story she told Cat in Episode 2.
I was confused by this, so I read the recap for S01E02 on the Game of Thrones Wikia, which stated:
Cersei visits Catelyn Stark, sitting by her son's side, and tells her that her first child, a beautiful, black-haired boy, died of a fever. She and Robert were grief-stricken and her prayers and tears were for naught.
In S05E01, When the young Cersei visits Maggy the Frog, Maggy only mentions Cersei having three children:
Cersei: Will the king and I have children?
Maggy the Frog: The king will have 20 children, and you will have three.
Cersei: That doesn't make sense.
Maggy the Frog: Gold will be their crowns. Gold, their shrouds.
Why doesn't Maggy the Frog mention Cersei and Robert's child in her prophecy? Does this omission prove the prophecy false? Alternatively, could this just be a giant plot hole?