Mushroom Gorge has one of the most intricate and iconic Ultra Shortcuts of all time. To pull it off, just drive forward a bit, turn around, and bounce off of a mushroom springboard. If you tilt your vehicle backwards in the air, you will be able to drive along the side of the mountain, go behind the finish line, drop behind it, drive to it, and complete a lap. This works due to a bug exclusive to mushroom trampolines. The momentum you gain from jumping off the mushroom is negated only if you touch the ground. If you hit a wall, your momentum is conserved, and if you hit it at the right angle, you can drive on it. But the game should have not let you get the lap because it should have recognized that a full lap was not driven. So why did it work? Well, the failsafe that was supposed to stop this was if you went from the box right before the finish line to the one before it, the game would recognize that you were going backwards and not count an extra lap. But the Mushroom Gorge avoids touching ground, and therefore you never enter those regions, skipping everything. The cut looks like this:
Similarly, Wario’s Gold Mine has an Ultra Shortcut. There is a pipe that was thought to be a way to skip the anti-shortcut failsafe I just mentioned. However, the pipe is out of bounds, meaning that Lakitu will pick you up if you hit it. However, hitting corners at high speed at weird angles does weird things, so it was possible to hit a specific point to avoid being picked up by Lakitu for a single frame. If you do the bounce at the right angle, you can go under the bridge that conveniently doesn’t have a collision check because you aren’t meant to be there. You go right through the bridge and turn around, crossing the finish line. If you miss the pipe, you get put back on the track by Lakitu, losing a few seconds, but the cut still works. It looks like this: