I keep a lot of background information on my many character and I make every effort to keep all the little details of their fictional lives. Unfortunately, I’m not perfect. Every once in a while an inconsistency creeps in, and when I write a lot of books in the series, that inconsistency can grow and take on a life of its own.
In the series Senta and the Steel Dragon, Senta has two cousins named Didrika and Ernst. The appear as babies in book 0. They get mentioned in book 5 and their ages had somehow grown to five years apart. When they arrive to play a bigger part in book 8, they were still five years apart but somehow had switched places. Didrika had been older orginally, and now she was five years younger! Holy crap!
I had to go back through the books and correct them. This was a small matter of changing a few dates in most place, but in book six, I had to rewrite an entire passage. Well, now it’s all fixed and thank goodness, because they both play a larger part in Book 10, which I’ll start writing next year.