Ha! That kind of thing will sneak up on you. I've had characters change numbers of siblings, change age by literal decades (fantasy will do that to you), and on one notable occasion, had a main character's seldom-used first name change.

Trust me when I say I've learned the hard way to keep track of EVERYTHING.

A classic.

There are few things more thoroughly sourced and comprehensively written than a fan-wiki for a series with an enthusiastic audience. Here's hoping that we all find ourselves with fans ready and willing to help build the resource.

Yeah, even for pantsers, the resource is valueable. It just switches from being a guideline to what you will write to a summary of what you have written.

Hey, I get it. I STILL struggle to remember to do a story bible, and I've drafted blog posts/advise emails about their importance at least twice.

It's hard to overestimate how hungry some fans will be for deep, detailed, clear rules and history of a world.

I'd completely forgotten about mind mapping apps. I haven't used them much myself but they are very well suited to this.