Fixing and endpaper accident
Yeah, who doesn't know it. You finished casing in, all dried nicely. Then endpapers are square and don't have skew or a wrinkle and when you take out the moisture barrier, turns out you damaged the endpaper a tiny bit, glue seeped in and you tear off some paper there.
Not a big deal with coloured through, papers without a pattern. You just clean it out, smooth the rough paper a bit down and you're good.
It's a different thing with patterned papers like mine here. In this case I was lucky, the pattern was not a complex one and on top the torn off piece was large enough to carefully peel away and just glue into place. There was a tiny bit of the pattern lost and my gaze kept stopping there. I just could not unsee it. So I checked my pens to see if I had a blue pen that was close enough to the colour of the pattern (I did) and carefully recreated the pattern with it.