To word this better, once a thing has happened, it is much easier for it to be done again.
Similiar to a mantra in Discworld; once things have happened, they have a tendency to happen again. The probability of an event increases slowly but steadily, until Rocks Fall, Everyone Dies or it approaches 1.
Compare So Last Season.
- Dragon Ball Z:
- The legendary Super Saiyan transformation was seen as impossible to achieve, yet so powerful and dangerous that it merited the destruction of the Saiyan homeworld and the near-genocide of their race. It took Goku, who was prophesied to get it, amazing emotional and physical stress to achieve it. Ditto for Future!Trunks, Vegeta, and Gohan. Then they discovered that there are levels to the Super Saiyan transformation. Then Chibi!Trunks and Goten, Vegeta and Goku's pre teen children, discovered they could transform at will, just because.
- And then (in Dragon Ball GT, at least) they discovered that the key to the truly legendary transformation (the one that warranted the genocide in the first place) were the Saiyan tails that Goku and Vegeta had long since gotten removed as liabilities.
And [one universal law] is that, no matter how hard a thing is to do, once it has been done it'll become a whole lot easier and will therefore be done a lot. A huge mountain might be scaled by strong men only after many centuries of failed attempts, but a few decades later grandmothers will be strolling up it for tea and then wandering back afterward to see where they left their glasses.
- The concept is referenced in Maskerade:
- In Monstrous Regiment, the main character is a woman. However, it is gradually revealed that apparently so is everyone else in her squad, with the exception of the nebbishy lieutenant, and including the extremely auspicious Sergeant Jackrum and a third of the high command.
- In Men at Arms, it was mentioned that only a genius like Leonard da Quirm could have invented a weapon so radically different from everything else Ankh-Morpork had ever seen like the gonne, but once the gonne was invented, all it would take to make more gonnes is a clever craftsman, which the city has plenty of. Bjorn Hammerhock was murdered by the gonne specifically to prevent him from reverse-engineering and reproducing the gonne.
- The Ellimist Chronicles establishes that while the odds against the Ellimist's ascension were tremendous, the odds of it happening a second time, to Crayak, were very good.
- Invoked in-story by one of the characters in Jacek Dukaj's Black Oceans.
- Star Trek: Department of Temporal Investigations gives this as an explanation for all the time-travel starship crews run into. Once you've done it, the chances of encountering it again increase, due to becoming a Weirdness Magnet.
- Ancient Domains of Mystery:
- This is a game mechanic. Items that you've found once are slightly more likely to be generated in the future. This is partly to make it so that spellcasters can actually use their spells without worrying about permanently running out of their good spells before finding a new book.
- What are the odds that a jackal kills you? Depends how many you've killed, since killing monsters makes other, future iterations of that monster slightly stronger.
- In Resonance of Fate, Bonus Shots work like this. If you attack an airborne enemy, you might get to roll the Bonus Shot chance, and your ability to succeed on the second shot goes up with every bullet you hit any airborne enemy with, stacking until the end of the fight or until you actually make a Bonus Shot. Also, after you actually make a Bonus Shot, well... your enemy's already in the air, so if you've still got actions left, just keep shooting and start building your chances up again.