The further I get into my story, the more inconsistencies I write myself into. I don't think it's anything that can't be edited out but it's frustrating, because they keep cropping out. For example, Person A agrees to do something because of coercion, then twenty pages later, Person B says they Person A did it because it was the "right thing to do." Easily edited in this case by changing just one line of dialog, and I know exactly where 90% of the inconsistencies are, but it's annoying. Are things like this going to happen, or is it a sign of something seriously wrong with the plot? I have a general outline and the whole thing plotted out, for whatever that's worth.
I'd go for something in between the two options - no, things like this aren't necessarily going to happen, but also no, it's not a sign of something seriously wrong with the plot. It sounds like you're working through your characterization, more than your plot, as you go. Totally valid way to write, and most/all writers do it to some extent. So, yeah, there may well be times when you need to go back and change something earlier. Not a big deal. In terms of not interfering with your forward momentum, you probably want to (if you haven't already) figure out some sort of note-taking mechanism for yourself. When I change something that will have implications on things I've already written, I use the Word "Insert a Comment" function to note precisely when things changed. For example, I might write "Biff made more sympathetic from this point on - rewrite earlier Biff". So my inner perfectionist is satisfied that I won't miss the change when it's time for rewrites, but my inner creator isn't distracted from moving on and continuing with the writing. You could do this with in-text notes to yourself, too, but they can be harder find later on.
For me, it's inevitable. I can't plan every little thing (I don't think anyone can) so I'm always going to change elements of the characters and therefore the plot as I write. Echoing @BayView that the best thing to do is find a way to note the inconsistency and move on rather than constantly back-editing. I can't say I've mastered this yet, but I'm trying!
I think this is the 'planning vs pantsing' debate again. The more you plan the fewer inconsistencies crop up when you write, but would you waste more time planning than rewriting? It probably depends on the writer (where they sit on the planning-pantsing spectrum: how decisive they are; how much wiggle room they like to leave themselves) and the story (how complex the plot is). I echo @BayView and @Tenderiser 's sentiments: if you've already started writing, just make notes to come back to later (I also use MS Word's 'Comment' function). You could create an index document or spreadsheet with a list of problems and potential solutions, and add to it as thoughts occur. If you find problems you're not immediately sure how to deal with, it might be worth planning again before you start rewriting. I'll show you! (In 50 years or so.)
