C^ hyperspeed
The C^ hyperspeed trick is a glitch that allows Mario to build up unbounded speed by sliding down a slippery slope while trying to enter first person mode.
Execution
To do the trick, you have to try to enter first person mode by pressing C^. Usually Mario's speed decreases and he comes to a stop, but if he is on a slippery slope he will accelerate. If he is facing uphill, he will get negative speed, and if he is facing downhill, he will get positive speed. If he is facing exactly 90° to the slope, he will still gain negative speed.