Accident in the Great Hall

When a young Anna and Elsa were playing with Elsa's icy magic in the Great Hall of Arendelle castle, an accident occurred in which Elsa unintentionally struck Anna with her powers.

History
One night, Elsa was roused by her sister Anna, who wanted to play with her. Once Anna had convinced Elsa to come and play, the pair went to the Great Hall, where Elsa used her magic to create a winter wonderland. After a lot of fun, Elsa created mounds of snow to catch Anna as she jumped through the air; but Elsa became overwhelmed, asking Anna to slow down. Suddenly, Elsa slipped, and in an attempt to break Anna's fall, accidentally struck her in the head. Anna tumbled to the ground, unconscious.

Elsa alerted her parents, Agnarr and Iduna, who rushed to the Great Hall. With Agnarr's initiative, the family rushed Anna to the Valley of the Living Rock, where Agnarr pleaded for help. Grand Pabbie stepped forward and proceeded to heal Anna while also erasing her memories of Elsa's magic. Grand Pabbie also explained to Elsa that she must learn to control her powers, warning her that there was not only beauty, but also danger.

Agnarr decided the castle gates would be locked, the staff reduced, and Elsa's contact with people—including Anna—would be greatly limited, all in an attempt to keep her powers hidden until Elsa was able to control them. This began the isolation of Arendelle castle.