One thing I only noticed last night, is that some npc's respawn even after you kill them. Take for example, the old man sitting on the porch at Emerald Ranch. After taking exception to his rather belligerent attitude, I decided to hurl a stick of dynamite at him. This resulted in the loss of all of his limbs; so obviously dead. I returned 72 hours later only to find him alive again. Makes me wonder if he's integral to a hidden side quest or something.
There are a few like that