vicissitude - vih-SIS-ih-TOOD - expected or unexpected changes (especially in one's life)
Example: "We make careful plans for our future, but there are vicissitudes that can change them overnight."
As a starter: and as a former teacher in English, I hope to add to reader's vocabulary by presenting a word a day in simple terms that anyone can understand - from current use in publications, radio,TV, etc. Remember: use a new word three times in a day and it's yours forever!