I sometimes forget things when I’m writing.
I set out to write stories with grand ideas of all sorts of content I want to put in them. I write in events, that are foreshadowing other events further down the line. I insert these things to allow me to use them for various stories and whatnot further down the line and most of the time I use them. Sometimes however I forget I put them in, I get to the end of the story and forget I put it in and realising it was too big a thing to ignore until the next book I have to go back and take it out.
In Brogan and the Bandit King, I had one of the characters retrieve something from the wheel arch of a wagon they had been riding in, only to not do anything with it later. Sometimes they are small enough things that I can ignore them for the time being and maybe use them later. In fact, it has to be said that I feel some of my best plot points and stories have come from overlooking a plot point in a previous story, before remembering about it just as I launch into a new one.
I like to call them happy accidents, most other people call them oversights.
This was a daily prompt response.