A vagary is an unexpected and unpredictable change, and the word is usually used in the plural. You might know from experience that the vagaries of winter weather make planning a vacation in February a risky proposition.
Vagary traces back to the Latin root meaning "wander," and you can think of a vagary as something that wanders. Events or situations that seem to change at random have vagaries. Whether the hottest new boy band will succeed or not is dependent on the vagaries of teenage enthusiasms. The vagaries of the real estate market will determine whether that swamp property you just bought is a gold mine or a disaster.