Urban Legend of Zelda: In World 5-1: Shy Guy Jungle, one of the things Mario can find in the pile of trash is a serial key: "XD3R-B8HH-9ZR2-FL16". Some say that this is an actual serial key for something, but what it's for is unknown. It's not a Club Nintendo or eShop code, because both display the "wrong code" message instead of the "code has already been entered" message.
What Could Have Been: At one point in time, as early trailers and screenshots showed, partners were set to make a prominent appearance (including new partners that had never been seen in the series, such as a Toad and a Chain-chomp), and combat was going to be more like previous installments. However, partners were removed and combat was overhauled to better accomodate the sticker mechanics.
Unique mini-games were planned for Bowser's Castle, but were cut due to time constraints during development.