The magic mirror that shows only ugliness, the mirror shattering, a shard in the eye making you see only the bad and making you mean towards your dear ones. Those plot elements are all in "The Snow Queen" by Hans Christian Anderson. In that story the enduring love of a forsaken dear one melted the mirror shards.
For the scene where the Snow Queen arrives in Boston, a sequence was filmed where Ingrid is seen exiting the magic door in the middle of the city. This ended up being cut from the final episode.
The title of this episode was announced by Adam Horowitz via his Twitter account on September 25, 2014.