When I wrote the second novel in my fantasy trilogy, I had a couple of dragons flying somewhere together, taking their riders somewhere deep into the enemy's territory. I had just about finished the novel and was doing some editing when something about that scene began to bug me, but I couldn't figure out what it was, so I marked the scene in red. My editor (my daughter) read it and asked, "Didn't Jorath die in the first book?"
I had a similar thing happen. At the end of series 1, I had a whole heap of characters move to another galaxy and the door closed permanently behind them. In series 2, I used a lot of them.
In series 4, I went looking for secondary characters to use, and assumed because one of the main characters didnt do the move, the 2nd level character also didn't.
WRONG.
I picked up the error 3 books into series 4, when I did a re-badge on series 2, and found I'd now used the same character in 2 different galaxies at the same time.
Since I was editing in the series 2 books at the time, I did a very fast retcon, and replaced the character used in series 4 with 2 other characters not used at all since series 1.
It is so easy to do something like this, even with documentation in place.