in the original snow queen story there was a mirror that broke into shards that was pretty important
Yeah, it was a mirror that reflected things as only being evil, then the mirror broke and became tiny shards which got into the hearts and eyes of people all over and made them only see and do bad. Then the little boy in the story got a shard in his eye and turned against his best friend and ended up in the custody of the Snow Queen.
(When he disappeared, everyone figured he’d died in the river. One of the few faint hopes I have for this season is that they make Will relevant by tying his (supposedly?) drowned-in-the-frozen-lake sister into the Snow Queen story.)
Eventually his best friend finds him and her tears melt away the evil mirror shard in his eye and yay happy ending.
There’s another mirror reference too, the Snow Queen calls her frozen lake the Mirror of Reason. When they brought Sidney back, I figured there was gonna be some parallel whatever between Regina and her bad Mirror and the Snow Queen and the mirrors in that story.