Minecraft

Spontaneous combustion is a form of world instability, created when a world is made with missing, too many, or clashing components, or illogical aspects.

Think of instability like this: does this world make sense? For example, if you make a world tiny biomes and huge biomes, the two clash and some instability is added. Some attributes automatically are unstable, like dense ore worlds. The more clashes, the more bad stuff. It can range anywhere from eternal slowness or blindness, being caught on fire when exposed to the sun, to world-crushing meteors and ever-devouring ooze that destroys the entire age.

Ah, okay, I knew about instability but not that fire was a kind.  Also, I've changed my mind: instead of FtB, I will be using my own slightly similar set of mods:
•BuildCraft
•IndustrialCraft
•ComputerCraft
•Additional Pipes
•Rei's Minimap
•Secret Rooms
•MystCraft
•Sneaky Pipes
But anyway.  I feel like I've been boring everyone with my rambling about FtB so I'll try not to do that as much anymore :/