I love the randomness of STALKER. I was playing CoP complete and I was inside this bandit base that was neutral. I'm using the setting that turns people caught in blowouts into zombies rather than killing them, and there had just been a blowout. Then when I... uh... made the bandits hostile, they were simultaneously attacked by stalker zombies that were wandering nearby that noticed the bandits shooting. So they were distracted and things were made easier for me.

Not to mention every time a blowout actually happens, always interesting.
Heh the timing was good.
I recall the first time playing Stalker, I made the military south of the starting village hostile because I got too close to them. After being shot at, I ran back to the village and carried on my duties, accepting jobs from the Trader. While managing my inventory, I hear multiple gunshots and explosions. As I go the surface, the village was attacked by the military who shot at me. It was a pretty cool battle...and I got to keep everybody's loot when they were dead.
The military outpost was now empty as all military soldiers had attacked the village. So I was able to loot the place and take a quest item much before I obtained the quest itself.

Another cool event I had with Stalker, was using the L.U.R.K mod. (I now use Complete)
I was returning to the Trader for one of the main quests. As I arrived in the Cordon from the north, there was a huge thunderstorm starting. In LURK, you can't carry much and I was already overloaded with loot. LURK also adds more enemy patrols.
After I got past the bridge, I heard multiple gunshots. I had no night vision so I had to use my flashlight. As I'm looking with binoculars where the gunfire comes from, I hear someone running towards me on my right, breathing heavily. I turn back and I fall face to face with a Bloodsvcker. I froze there for a second or two before I shot it. ^_^ The bastard, wounded, then ran back in the darkness and I did not pursue it.
So I'm now wounded, I don't have any medkits left and I still haven't made it to the Trader. While walking on the road, still hearing the gunshots on my left, (They came from the Bandits in the garage) I use my binoculars to see if targets are in front of me.
I see 8 red targets coming towards me. When there's thunder flashes, I can see 8 silhouettes in the distance. As they approach, I then see their 8 flashlight lamps.
I jump over a railing, lose most of my energy and hear the bandits screaming and shooting at me. As I tried to walk back to the Trader, I also got attacked by two boars. I managed to arrive in the village with barely enough health to survive, hungry and fatigued. All of these events were even more awesome because of the thunderstorm and Stalker's excellent weather system.