Basically, this refers to an issue within a story where payoff exists without setup. I’ve heard it also described as ‘a sudden solution to a problem within the story’ (paraphrased), though I don’t fully agree with that.
I think my definition is fairly straightforward.

This will be the only example listed for a while, because I just can’t think of more of them right now.
In Sword Art Online, Kirito being able to prevent Yui from being deleted – and then consequently turning her into a game item – is a plot hole, since there was zero prior indication that he had the skills to do this. Unless, somehow, you consider him being a nerd adequate setup.