the spark that caused the Hindenburg combustion.
the iceburg that sunk the Titanic.
the meteor that wiped out the dinosaurs.
the drought that triggered the Dust Bowl.
the crash that kicked off the Great Depression.
the volcanic eruption that buried Pompei.
the rats that carried the bubonic plague.
Your suggestions are welcome!
After this was first posted, the CB Bucknor Twitter account had this to say: