SPOILER: The mystery upgrade makes it so that when your plane touches the ground it transforms into the turtle from Toss The Turtle, the penguin from Learn To Fly, or the hedgehog from Hedgehog Launch. It also stop your boost and makes you bounce along, rather that stopping right away
 "Epic Friends"){kind=icon}
 "The Company of Myself"){kind=icon}
 "Flight"){kind=icon}
 "Papa's Freezeria"){kind=icon}
 "Music Catch"){kind=icon}
 "Achievement Unlocked"){kind=icon}