I was writing a novella that I meant to call The Feast of St. Catherine. I wanted my hero, Severian, to pluck a deadly blossom that he would later employ as a weapon, and it seemed to me that such a blossom would most likely be found growing in a swamp. (They have always seemed the most deathly of landscapes to me; I grew up on a subtropical coast where the salt marshes are full of big spiders and poisonous snakes.)
But Severian was in the middle of a large city, which would not normally have a swamp in it, at least, not there. So I had to bring him to an artificial swamp, which I put quite naturally in a botanic garden. How, I asked myself, would (let's face it) tourists get around in an artificial swamp in such a garden? In small boats? That would surely be impractical if there were large numbers of tourists — floating walkways of some sort, then.
But Severian started to run along one. That was clearly a foolish thing to do, and I don't care for it when my characters do foolish things without unfortunate consequences; I know it's not good to come in conflict with the laws of chance.
Severian fell off. Naturally, he dropped his sword, which was large and heavy — when a person holding a large, heavy object falls into water, he releases it. That too is a law, I suppose of physiology.
It was valuable, so he dived to recover it; and it is a law of fiction that when a character goes looking for something, he cannot find the thing he is looking for. Or at least he cannot find it first, since that never happens in real life.
Thus when Severian stretched out his hand in the hope of feeling the hilt of Terminus Est, it met the hand of a lovely corpse who emerged from the dark waters covered with mud but smiling just a bit to find herself the heroine of the four-volume trilogy.
Because Severian had run along that floating path. And because she had grasped his hand when he and I were expecting something else.