I need a word for when an event or situation that is very unlikely to happen, happens. For example, let's say the nerdy kid in school, who is an outcast with no friends, dates the popular girl in school. From a stereotypical view point, the likely hood of a "nerd" to date the popular girl in school is very unlikey, but what if it did happen? Is there a word for this type of scenario?
not ordinarily encountered
Try "black swan":
black swan n
2. a phenomenon that occurs even though it had been thought to be impossible
[sense 2 from the fact all swans were thought to be white until black swans were discovered in Australia]