It's possible that I'm reading too much into this but I've noticed that sometimes the NPC's are very specific about what they say. Meaning that they are surprisingly capable of commenting on events around them as they happen. to give a few examples.
A walked into a guard kiosk, and decided I would have a little fun pretending to intimidate the guard. I showed him all of my weapons to see if they would garner a reaction out of him. He simply stood there staring at me. So I proceeded to pour out the gasoline in my jerry can. At which point him and two others ((who were very well hidden as they seemed to come out of nowhere)) opened fire on me, killing me. Not terribly interesting except for what the guard said as I laid in the floor bleeding to death.
"I won, you lost, you should have walked away"
another time, I tried to follow a player down an alleyway with my blip off the radar and a knife in my hands, with the intention of stabbing them to death. Sadly the cops got to him first, so I paused the game to check the map, only to hear the following.
"I will smack the sh*t out of you!"
followed closely by
" I don't play games asshole"
Upon unpausing the game, the cops were royally pissed and attacking me, even though I had not technically broken the law (as far as I know)). I did not attack them first. But I did kill mr. smack the sh*t out of you before they took me down.
And lastly, though I'm not entirely sure that it's not more common than I think it is. I just got done with an airport shootout between myself and 5 other players. Clearly the cops were on the scene in droves, though I seemed to be the only person killing them. I hear one of the officers say.
"this is on the news pal, you're f*cked"
and you get the idea. It's a bit eerie that they give the impression of being that aware of what is going on but I just chalk it up to rockstar having worked on this game for a very long time.
I am however very interested in hearing from other people about this subject. It could be funny, it could be scary, just make sure it seemed to fit the context of what you were doing at the time.