Ok, for me all the time, but I just noticed it this time because it was different from before.
When I was writing Frankenstina, the outline that took me 3 months to prepare was driving the first 15 chapters of the book. Then all of a sudden, the book seemed to take on a mind of its own but it stayed within the overview of the main outline.
One thing that happened was it created the 3rd book in the 4 book series. I had only planned on writing two books. Frankenstina and Frankenstina Reborn. But at some point, events started happening in the book that even though it didn't leave the reader wanting more info, it did present the opportunity to create another story line. If I had tried to include it in the first book, instead of it being 413 pages it would have wound up being over 800 pages. And I was working on a deadline so that was impossible.
This also happened with Training a Titan I. I had only planned to do one book of this Young Adult adventure book, but in the very last chapter the book decided to create a cliff hanger. OMG where did that come from I asked. Sooooooooooooooooo....I got to work drafting Training a Titan II "Circle of Wolves"
In this sequel a minor character has now become a supportive character at page 251!. He couldn't become a main character because he wasn't going to be around long enough. But the book needed him in order to facilitate two of the main characters ability to function believable to the reader.
How's that for not giving a spoiler alert?
Basically what I'm saying is, no matter how detailed you made your outline, or how clear your vision is for your book, if you write from the creative mind, the end product will tend to be a wee bit different.
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"Throwing Stones at a Glass House"