I love this, I am guessing you have played space invaders before and you are aware how the game works.
As you shoot the invaders they gradually get faster and faster. It was a nice game play mechanic as it made the level progressively harder the closer you got to finishing it.
Well apparently this wasn't ever actually programmed in! In the original game hardware struggled to keep up with all the graphics on screen, as you removed some of those assets the hardware was able to render stuff faster and so things started to speed up.
The hardware that he had assembled was not powerful enough to display graphics in color and the enemies moved slower than he had intended. Fortunately he discovered that the more enemies that were removed from play, the faster the processor could render those that remained. Rather than attempt to design the game to compensate for the speed increase, he opted to leave it in as a challenging gameplay mechanic.
This certainly has to be one of my favourite - It's not a bug, it's a feature! moments.