The ending of the game, which has all of Bowser's children escaping in an airship. Bowser, who's still tremendously huge, tries to jump on, only to bring the entire ship down. They eventually all escape in Bowser Jr's incredibly small Koopa Clown Car, with Bowser Jr. being the only one inside, with the rest hanging off the side. Including Bowser. The Junior Clown Car itself looks very unhappy to be carrying all the weight.
Luigi during the intro, Luigi during the title screen easter egg... heck, Luigi in general.
On occasion, the last player who enters the Koopaling's Airship will bounce off the one ahead of him while the rest will gently land...
During the opening, as Bowser and the Koopalings are attacking Peach's castle, a giant mechanical hand emerges from Bowser's airship, comes down to where Mario, Luigi, Peach and the Toads are having tea time... and gently moves Peach out of the way before sending the others flying.
After taking down Bowser in the final battle, Bowser Jr. will commit suicide by jumping down on his own.