I have a fun moment to share.
Playing Fallout 3, I decided to explore the west end of the map. First time playing and I discovered the eerie and notorious Dunwich Building, known for 'bad mojo', after first traveling south through the crevice due only a few meters east of the F. Scott Key Trail & Campground, the Campground being located north-west of the Dunwich Building and the crevice located north of the Building.
After completing my scavenging of the ruins, I emerged onto the night-fallen Wasteland and proceeded to hook west of the building and enter the campsite to the north. Calm so far, and everything seemed to be going swell. Until the weird sequence of events occurred.
I noticed a mole rat charging at me as soon as I arrived at the campgrounds, so I whipped out my trusty hunting rifle, a well maintained one at that. Mole rats were simple pests at this level threshold I had reached, and this one was no exception. Of all creatures of the Wastes, Mole Rats were now easy kill at this point.
Before I could fire off a shot, a Yao Guai spontaneously appeared from my peripheral vision, and I focused my attention on the larger, more threatening target on instinct. The Yao Guai had targeted the Mole Rat and had killed it in a single swoop. I knew the hunting rifle was no match against the brute, so I switched to the reliable SMG, but not without being penalized by a fresh hit from the powerful creature. I was at half health, and I took caution by using the environment to my advantage by jumping over some chest-high rocks nearby in the hope it would slow it down.
Then the amazing happened.
A Deathclaw appeared out of my peripherals, like the last creature had, and targeted and killed the Yao Guai in two swipes of its powerful claws, then began to charge at me at full speed. I knew my only option at this point was to run and no weapon I had with me at that moment would be able to effectively take it down before it would tear me to shreds. I was involved in a crazy hijinks of an incomprehensible caliber where I knew I was going to die, but I ran anyway. All that was going through my head was "I am royally screwed" while laughing heartily at the events that had unfolded before me, and I was too shocked to even shoot.
Of course, the frosting on the cake came when I was smacked by the Deathclaw in the face while moving backwards in an attempt to gain ground, and of course, I died.
A short rag-doll tumble down a hill, a disengaged arm rolling alongside my body, the death of a good, yet deadly (Not anymore XD) samaritan, and me laughing very hard while looking back to what just happened are the end result of 30 seconds of silliness.
I love this game.
EDIT: After a little digging around, I now realized the Mole Rat, Yao Guai, and Deathclaw do indeed spawn there in a scripted event.
http://fallout.wikia.com/wiki/F._Scott_Key_Trail_%26_Campground
Still, I am proud to say this is one my favorite 'spontaneous' videogame moments.