On Wandering Monsters and Unplanned Encounters

Jay "Rampant Coyote" Barnson has written down some interesting thoughts on random/unplanned encounters in RPGs and how to handle them. Here's an excerpt:
Unplanned combat encounters force the player(s) to play defense. And not usually with a nice fortified defensive advantage, either. The player may be caught unprepared. It may disrupt the best-laid plans.

Having played a lot of 3.x edition Dungeons & Dragons, I became fascinated by how the game mechanics made such such a difference between the players attacking a dragon and the players being attacked by a dragon. Given the opportunity for adequate preparation particularly attacking a dragon in an underground area that inhibited its mobility, and preparing magical defenses against the dragon's breath weapon attack a combat with a dragon of the appropriate challenge rating was a difficult venture but not exceptionally dangerous. But the same dragon attacking the party out in the open when they haven't had time to prepare? Whole '˜nother story, and often one that resulted in one or more characters dead or unconscious by the end.

Individually, surprise patrols or ambushes or chance encounters aren't the kinds of things players relish (though it may be something they seek out, when grinding for that last bit of XP, gold, or some other loot they need). I remember some such encounters causing me some serious frustration in the Gold Box days, where a chance encounter could mean another 20 30 minutes (or more) of tedious, repetitive combat. Ditto for Wizardry 8, where I actually timed the combats at sometimes nearly an hour in length even with the third-party combat speed accelerators. I don't think combats should last that long anyway, but when they are thrown at you when you are trying to do something else, they can be infuriating. With a few exceptions (usually story-driven, like the above dragon attacks), unplanned encounters should not be major events.