Upward Spiral without any powerups. Every slope is made out of Random Force. The Qualify Time is 20 seconds, and there are TMs set to -60:00:99 everywhere. The TM shapes are also 4 times their original size. There are fans with blades that can actually hit your marble sending it OOB. If you manage to get to where you think the finish pad is, a help trigger will tell you that the finish pad is actually hidden near the beginning of the level. But once you touch the fake finish pad you trigger a bunch of walls that trap your marble, and you also trigger a TNT block rain (like in Minecraft) which destroys progressively your marble, and the amount of the explosions will lead the game to display a trollface before crashing. But it only causes visual corruption, and you must manage to lodge yourself into a block, where you are shrunk down to a thousandth of the marble's size, and must climb the platforms by the pixels in the textures. This will make the whole game lag as never. You must then find the invisible super-checkpoint, which you must get to save your progress, in order to restart the game and escape the lag. When you escape the lag, you'll find yourself in another dimension, in which everything is dark, there is more lag, more TNT, and finally you find that the game crashed when you touched the TNT. When you reopen it, it acts like Hell.. o? Now the marble is a serial killer and destroys your computer. To protect your computer, you must agree to a lengthy EULA that if you have acted with poor behavior, will sacrifice all future family members to the grave. Else, you must use the GarageGames marble and beat the level without TNT, but with random bumpers falling out of the sky to right next to you, every three seconds. The bumpers will fly and explode on contact, not to mention they follow you. Besides, you'll get an increasure of 20% Warning level when you collect any of the 15 gems. But somehow if you hack the forums and reduce your warning level back to 0, you can finish... except that the finish pad is surrounded in a teleport trigger that takes you to a secret part of the level with tightropes moving at 200 units per second.
QuArK is still a good map editor
You know what's boring? Opaque marble skins. You know what's not? Glass Marbles!