After two more serious posts in this series, I wanted to stop somewhere a bit more lighthearted and inconsequential.

“Here’a a fun thing you can try,” begins this 13-minute video from Retro Game Mechanics Explained:

The premise of the parlor trick the video describes is a bit convoluted:

• Get your Nintendo Entertainment system, put in the Mario cartridge, and power it on,
• without powering it off, grab the cartridge and replace it with the Tennis cartridge,
• start the game, wait until the music stops, then move around for a bit,
• take that cartridge out and put Mario back in,
• press A+Start and you will see that you will start at a random “invalid” level that doesn’t exist, which should not be possible.

I’ll let you watch the video for a nice explanation, but this I think is a good example of the Swiss Cheese model:

• A+Start is real feature that allows you to continue the new game at the last level you finished (as opposed to Start, which always goes to Level 1).
• The NES allows you to take the cartridges out and put them back in while the machine is under power.
• The creators of Mario decided to make the game reset itself on every game over, and differentiate between cold boot and warm boot in a specific way.
• Tennis doesn’t wipe the memory on start.
• Tennis happens to change the memory in a very particular way that ends up being accidentally “compatible” with Mario.
• The Mario game doesn’t sanitize the level number on entry, so you can start on an invalid/​broken level.

Just like before, the absence of just one of these Swiss Cheese holes would prevent the bug from happening. But the holes line up perfectly.

I found this whole thing kind of fun and delightful, and I actually understood why the creators of Mario decided to reset the game (with a small exception) on every game over – it’s brutal but elegant, and something I found myself wishing was easier to do during various projects I worked on in the last few decades.

Of course, there might be an argument that this is such a highly manufactured scenario (how did anyone even find that out to begin with!?), and of limited value, particularly if you end up at a random level anyway. We will pull on that thread in a future post, because the story doesn’t end here. People who spend time hacking old games go way further than this.