"Whatever happens, it's unexpected." As defined in *84 Charing Cross Road* by Helene Hanff. Said philosophically about unexpected turns in careers, especially in the arts.
Flanagan's Law can be a useful guide to preparation for challenges when combined with Murphy's Law: it's not just that if anything can go wrong, it will; it's that whatever goes wrong will go wrong in a way you didn't expect.
Helene Hanff, popularizer of the phrase, hoped to advance her future as a playwright or actress by reading classic literary works from a very old-fashioned reading list. She began to order books she couldn't find in New York from an English bookstore. It was her correspondence and friendship with the bookstore staff that made her famous: Flanagan's Law.