When in a level with thwomps, you can make them shake strangely, like in Bowser's castle or on the Rainbow Road. It's easiest to do in two-player match mode because then you have no time limit. Make one person go forwards all the way to the second lap. This makes the thwomps start stomping up and down. Then, go backwards through the level and go back past the goal tape a little ways. (In Bowser's castles, to go backwards, use the R button to hop over areas of lava.) The thwomps will now be shaking in the air erratically. This is probably because when you're on the first lap, the thwomps stay up while on the second, they go up and down. So, they don't expect you to go backwards and the game gets confused.
Another interesting thing to note is that when you go backwards for a long time, you have to go forwards and make up for all the area you went backwards and then go through the normal race. So, if you go backwards 3 laps, you have to go forwards 8 laps to complete the race. I don't know if this was intended by the programmers or really a glitch.