**All ages that are not fully written by you may be be unstable; prior to 0.10.4, this was guaranteed.**
Instability of an age represents a book that either does not properly describe the destination, or that describes impossible or illogical worlds. The representation in-game can be anything from an atmosphere that is harmful (in-game effect: debuffs that give you penalties; most of these are avoided by digging underground), to a world that is falling apart by spreading [[ages:instability#decay|decay]] or [[ages:instability#crumble|terrain crumbling to sand]]. Additionally, effects such as [[ages:instability#lightning|lighting strikes]], [[ages:instability#explosions|explosions]], or [[ages:instability#meteors|meteors]], may be seen as well.
Roughly in order of damaging-ness:
[[Ages:instability#black_decay|Black decay]] is a death sentence for most pre-generated lands in a world in the current system -- it causes sinkholes that eventually lead to the void. Specifically, the end result of black decay looks like skyblock survival (platforms and isolated sky islands survive).
Crumble is a death sentence for anything you try to build, or the naturally occurring ground and ores. Eventually, it will all turn to dust (sand).
[[Ages:instability#white_decay|White decay]] is also a death sentence for _ANYTHING_. White decay spreads through air, and will even consume a void world. White decay causes damage when you walk on it (not if you are standing still, and healing). Neither bedrock nor obsidian will stop it. The only known way to get this without altering the configs is with multiple dense ores, or really, really bad writing. \\ If you really want to play with this, write in 10 biome controllers, and have an escape book in your hotbar. Oh, and for safety, move at least 150 blocks away from spawn, just so that if you do die you can respawn. This is not a joke. (Good luck making it that far :-) ). For more on surviving in white decay, watch http://www.youtube.com/watch?v=0nZOgJkdUQI \\ As bad as white decay may seem, there is much, much worse.
Explosions are a death sentence for anything you try to place in the world. Explosions will not destroy bedrock, but can occur inside a bedrock lined room. It is impossible to place something, such as a bookstand with a return book, in such a world in a way that guarantees its safety. Eventually, explosions will erase the surface down to the bedrock layer.
Meteors are a death sentence for a world. If they are present, they will eventually destroy everything, including bedrock, even stuff you add as platforms. A descriptive book has to be written very badly, generally intentionally, to make this happen; random worlds will not have them.
Note that explosions and meteors are able to cause you damage; it is impossible to protect yourself from explosions, but as long as there's something above you, the meteors have to break it before they can reach you.
Version 0.9.4 has this basic behavior: - Descriptive books have a hidden internal value to represent instability. - Descriptive books that you write, that are missing "critical" logic, will have that critical logic added, and instability added. - Any use of certain symbols will increase instability. - Any use of certain symbols (representing some forms of instability) will decrease instability. Note that the amount of decrease is less than if that form of instability was randomly chosen. - All symbols used are visible, and can be copied. - Descriptive books cannot have symbols removed once written, nor any edits once visited. - You can take a descriptive book of an instable after visiting it, copy it to a new descriptive book, and visit that -- this will give you a nearly similar world (possibly with additional random symbols), usually with less instability.
Version 0.10.x uses similar behavior, but the symbols added by the game will not be visible in the book, but are rather saved to the underlying NBT data files separately from the written symbols. As such visit-return-copy will no longer work.
0.10.4 and later use a [[http://en.wikipedia.org/wiki/Context_free_grammar|Context Free Grammar]] system to attempt to fill in incomplete books by adding required fields and attempting to utilize all dangling modifiers. This system also allows for partial/complete descriptive books to be generated as loot as well as unexpected features (Tendrils, etc.) to be added to partially written books.
As of 0.10.4, incompletely written ages do not inherently have instability; prior to 0.10.4, all incompletely written ages had instability based on how much was not written. In 0.10.4, insufficiently detailed ages will have random terrain features added; those random features may include things that cause instability, such as coal tendrils (one of the more common, and less harmful cases.)
Future versions may allow for rewriting of books to more completely describe ages and thereby reduce instability.