I wrote a cellular automaton in university (elementary game of life). It was generating shapes but it just didn't work. It seemed like the shapes would degrade as they went until they caved into some mesh of pixels.

Turns out that when counting the number of neighbours around a cell, I was counting from 2 because of an earlier bug.

The really interesting thing about this bug though was that it made sense. There were too many people and the resources ran out, which meant the patterns that would normally survive were dying off early.

It's a bug, but not a bug.