Living corpses in Fallout New Vegas

This question is aimed at you savvy types who know how video games actually work on the computer program level. Several times in playing New Vegas I have encountered live NPC’s who were supposed to appear in the game already dead. Most recently, Trash at the old atomic test site and the hooker at the Gomorrah who Clanden was supposed to have tortured to death were both alive and well. Trash would even respond to conversation on a very limited basis.
I’m curious what goes wrong in the game program that creates “living dead.”

It’s a virus.

It’s usually a simple error in scripting, typically of the sort where a developer forgets to update the script after an unrelated change.

I’m not familiar with those in particular, but I have heard of "Burned Corpse"s walking around because a flaw of their raider attack script. They’re set up as live targets with the script intended to deal a lethal amount of damage to them so that the bodies fall over in a manner consistent with other killed-in-combat bodies. The problem is that the amount of damage the script actually deals to them winds up being slightly less than lethal and voila - walking corpse.