When I was a teenager, I once visited an arcade (I think this might have been near Indiana University, of all places) that had a coin-op arcade machine called Marble Madness. The objective was simple -- roll a marble through a pseudo-3D maze using a large track-ball interface. It was great, and reminded me of the various pinball-like animations done for the Electric Company TV show years ago.
Later, I acquired the PC version (some would call this a "port", except it came out years later) of the same named game. It didn't have quite the same appeal on my Dad's monochrome Hercules graphics card PC-XT. The keyboard was the primary interface to control the rolling marble, which was hardly ideal. Nonetheless, I managed to find enjoyment in this. Exhibit A: Young Nerd with no life to speak of.
I accidentally stumbled across a secret level in the game, which according to Wikipedia is traditionally known as the "Water Maze", and was not available in the coin-op arcade version. It really was a strange accident. To uncover this level, you had to have the marble sitting in a particular location at a specific time. When this occured, part of the ground would slowly drop down, taking your marble to the fabled Water Maze. When I first witnessed this happen, I think I may have freaked out a little.
I could never beat the Water Maze, however. It was quite unforgiving. Any sort of mistake would immediately end the game, rather than the usual replacement of your lost marble with another. There's an archived write-up of the level here, complete with screenshots.
The write-up isn't entirely accurate, however. Based on my experiences, it is technically possible to get past the first screen without two players, at least on the PC version. If you rolled your marble onto the correct lily just so, it would take you to the entrance to the next part. I do admit, however, that once I got that far, I was stuck. I could never successfully take my marble down the elevator without instantly ending the game.
Apparently nothing particularly interesting happens if you did manage to finish the Water Maze, however, so I guess I didn't really miss out.