In the 1950s, when the European Union was just coming together, not every country was so eager to join. The people pushing for it, like Kimmo Kiljunen, a member of parliament from Finland, had to do some convincing. There were years of negotiations, each one filtered through tedious layers of translation into over 20 languages.
So by the end of it, to close the deal, Kimmo and others added a law to the E.U. constitutional documents: Article 50, an exit clause. Kimmo never thought anyone would use it. It was meant to be a way to make skeptics worry a little less.