So in a homebrew/pathfinder campaign I started recently, the party (consisting of me an elf rogue, a human mage and a dwarf fighter) was sent to a near by town to do some work for a bounty hunter. On one random encounter we fought a small group of goblins. Our mage decided to teach himself the spell “animate undead” and used it on one of the goblins. Later we came across a couple of orcs. One of which had his head smashed in by the other, and the other had been bitten by the undead goblin being controlled by our mage. The DM decided that it would take the orc 24 hours to get back to his village so he could tell his tribe why he had killed his fellow warrior by accident. And it would also take the same amount of time for him to turn into a zombie. So our mage had accidentally just created an entire tribe of zombie orcs by pure chance.