[morleytj]

One of the classic emergent behaviors (which I loved) that came from Dwarf Fortress was the cats dying of alcohol poisoning.

There was a patch note years and years ago about a bugfix that had to happen because all the cats were mysteriously dying in people's fortresses -- it was tracked down to the fact that cats would walk through the taverns, in which visitors would be drinking and occasionally spilling alcohol (There was a feature that had been added at one point for spilled liquids to form pools which could get on entities passing through, like getting mud on you by walking through a large puddle).

The cats also had a piece of functionality where they could self clean by licking after they became dirty with something, and would ingest some amount of it due to using their tongue to clean themselves.

The cats' fur would become damp with the alcohol as they stepped through the spills, and the cats self cleaning meant they would regularly get extremely drunk and die from the alcohol poisoning trying to clean themselves. Not intended at all, but two completely different systems colliding in an emergent behavior of interest.

I also always loved the behavior of undead zones. In them, any dead creature could be revived by dark magic in the area. It leads to the question though, what counts as a dead creature? Well, it would be anything with a tag indicating that it came from something that died. This does in fact include small bones though, or hair from a butchered animal. Fortresses in these areas would have quite the casualty rate trying to butcher a pack mule as its hair would come to life and kill the butcher.