A lot of people assume that 'A Mistake Novel' is one specific book, but I'm pretty sure that phrase isn't a standard title. The OP might be thinking of a particular work where the central conflict revolves around a character's single, massive error in judgment—the kind of story that gets labeled generically. I think the resolution for a plot like that can only go a few ways.
One common path is the character facing the full, brutal consequences. It's the 'no take-backs' ending, where the protagonist's life is genuinely altered for the worse, but they achieve a sort of grim self-awareness. It feels more realistic and sometimes more impactful than a tidy fix. Some readers hate this because it's bleak, but I find it leaves a longer shadow.
The other route is the redemption arc, where the character spends the back half of the book trying to make amends. The ending hinges on whether their repair efforts are accepted by the person they wronged, or if the point is that they change internally regardless. I'm often skeptical of these—if the mistake was huge, a neat bow feels unearned. A good one leaves the relationships still frayed, but with a thread of something new, maybe respect.
In my search for this, I remembered a book called 'Atonement'—which is literally about a mistake with lifelong fallout. Its ending is a meta-resolution, where the character tries to fix things through fiction because real repair is impossible. That kind of layered, unsatisfying but fitting closure is probably what the OP is digging for, more than a simple happy or sad stamp.