greatest hits ~ a question

So i have a question and i'm not sure if its been answered already or not i just don't remember. Did we ever find out what the acident Juliet was talking about in the Greatest Hits was ? she stated that no one has ever been down to the looking glass due to an accident. does anyone know what it was ? sorry if its so random but my friend and i were rewatching the episode at school today and we couldn't remember if we ever found out what the accident was.

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