In 1286 Alexander III was traveling from Edinburgh Castle to Fife in order to visit his Queen (against the advice of his advisers (seriously, why does nobody take the advice of the people whose job it is to give advice?)), weather conditions were treacherous his horse lost it's footing in the dark and it went over a rocky embankment taking the unfortunate King along with it. He broke his neck, and with all his heirs dead, excluding his two year old grand-daughter, Margaret.
Unfortunately, she died while on her way to Scotland to be crowned.
one thing lead to another and before you know it, Edward I of England has sacked Berwick.
Now, what if Alexander III had taken his advisers' advice and remained warm and dry in Edinburgh for the night?