If you buy bananas on trash day, all the fruit flies will find their way into your home because of people's trash bins being moved about. Much like "Don't tempt fate", this is a phrase used to call someone out for their own foolish mistakes, or to warn them that if something can go wrong, it likely will go wrong.
Bride: "Awe man! I spilled catsup on my wedding gown!"
Mother-in-Law To Be: "Well, that's why you don't buy bananas on trash day."
Mother-in-Law To Be: "Well, that's why you don't buy bananas on trash day."