In the first drafts of the next few chapters of Too Much Love, I've managed to (a) completely lose track of what day it is in the story (b) send Emily on the same car ride twice (c) put a significant supporting character in two places at the same time and (d) have Cricket declare she wants to take a gap year from Yale when she's already taking a gap year from Yale. Other than (d,) none of these require retcon, but taken together they make me feel like I'm writing fan faction set in my own story.
Some advice if you're thinking about writing a million-plus word soap opera of a novel:
(1) If it's at all possible to tell your story using a different structure, do not write a million-plus word soap opera of a novel. It will probably take you years to finish and every level of pre-planning (from seat-of-the-pants to careful outlining) has problems at that scale.
(2) Get a good editor or editors. Mine has saved me from an enormous number of continuity errors (as well as grammar issues, misspellings, bad wording, mushy phrasing, and a couple of dumb ideas.)
(3) Keep some form of TL;DR documentation for yourself that you can check back on to quickly look up names, events, settings, etc. The farther you get into writing a story, the more your time will be eaten up cross-referencing past events.
(4) Even if you have TL;DR (too long; didn't read) documentation, periodicially go back and reread what you've written. The ratio of time it takes to write a novel to the time it takes to read one is 10+:1 and can be a hundred times that.
(5) If you're posting as you write, keep backfilled changes (aka retcon) to a minimum. People keeping current won't know they're there even if you announce those changes and they take up a ridiculous amount of your time.
I'm near the beginning of a reread of TML now in order to do some forward planning, to try to remember all of the threads that need to be woven into the remainder of the book, and to update my continuity guide. It's not coming a moment too soon and there will be at least some retcon-necessary changes.