During a quick Fate session, two characters were engaged in a duel, during which an onlooker was hurled into the fray, resulting in one duelist gaining the boost Petticoats and Fluff on the other. Shortly thereafter, both characters moved to a different zone, without the boost being invoked.
At that point, according to the events of the story, being temporarily tangled up in another person's underwear isn't really relevant anymore (barring the amusing idea that they're being dragged around as the swordfight progresses); however, as boosts can be generated as rewards for success with style, it doesn't seem right to remove them from play without granting the bonus to a roll, but at the same time, the narrative may not ever justify using them again.
What happens to an unclaimed boost when the story changes to obviate it?