- FL grows up in a privileged background, ML grows up without his parents.
- FL's mom knows ML as a child before FL even meets ML.
- FL's mom does not like the ML for a mistake that the ML made as a child.

Similarities: Female lead will forget everything due to the betrayal of the male lead. The female lead will love the male lead after forgetting everything but will be the most vengeful once their memory returns.

Differences: Once Upon A Time is a film while Good Bye My Princess is a drama. The male lead of Once Upon A Time did not truly betray the female lead while the male lead of Good Bye My Princess truly betrayed the female lead.