The powerful flashback
Before season 5, Game of Thrones did not do flashbacks, and it uses them so sparingly now despite Bran’s powers. On one hand, it reduced the need to cast younger actors in an already-huge cast. On the other, it worked narratively. The show deals with the idea of perspective. Simply put, look at the different descriptions of, say, Rhaegar Targaryen. To some, he was a hero and a wise prince. To others, he needed to be cut down. When we finally see him, he is making a really bad choice for love.
We say all this because the prophecy that Cersei Lannister received as a child was so powerful that it got its very own flashback.
Anyway, the flashback actually tells us a lot about her and her motivations. Also, it shows us that even a younger Cersei has plenty of insults and entitlement already. The witch agrees to tell the fortune, and almost all of it has come true. She married a king. She had three children who all had golden crowns and golden shrouds. She has become the queen. All that remains to be seen is a younger, more beautiful queen to depose her.
Does Margaery Tyrell ring a bell as a younger, more beautiful queen, married to Cersei’s sons and thus a queen?
Yes, yes it does, or at least it does to Cersei.
This prophecy also probably predicts Daenerys’ victory, by the way, and might be behind the double-cross Cersei pulls in seasons 7 and 8.