My favourite instance of emergent behaviour in video games is cats dying from alcohol poisoning after eating alcohol-laced vomit in Dwarf Fortress, and the bug reports/forum posts trying to determine what was killing the cats:

http://www.bay12forums.com/smf/index.php?topic=154425.0

https://www.bay12games.com/dwarves/mantisbt/view.php?id=9195