When Jefferson takes Regina through the hat, they end up in a room filled with doors into different worlds. One door, close to the Looking Glass, is emerald green with two golden door handles. The handles are each a golden "O" with a smaller golden "Z" in the center, signifying that this door leads into Oz, the world in the Wizard of Oz story.
When the Queen and Jefferson walk into Wonderland, the Caterpillar, voiced by Roger Daltrey of The Who, greets them with the iconic line "Who are you?" which is also a song by the band.
Sebastian Stan (Jefferson/the Mad Hatter) and Jennifer Morrison (Emma Swan) were dating around the time of or after this episode.
The door inside the hat that leads to Wonderland is a mirror; a mirror (or looking glass) is how Alice returned to Wonderland in the sequel book, Through the Looking Glass, and is later a recurrent method of journeying to and from Wonderland in the spin-off of this show, Once Upon a Time in Wonderland (2013).
The outside of Jefferson's house is the same house used as Johnny Smith's house in The Dead Zone (1983), which also takes place in Maine.