I have a pretty highly abused reading plan that displayed some oddities when I fiddled with it today. It's a plan that I'll read for a few days then skip for a few months then adjust to the current date - rinse, repeat, etc.
So after several months neglect I thought I'd try picking it up again, so I opened it up and clicked to adjust. It seemed to be set up fine until I took a closer look.

Note that between Mar 2 to Mar 6, it jumps to later page numbers and has those pages marked as read before returning to the expected page range.
As I say, I haven't exactly been nice to this poor plan. It has been neglected and abused several times. Still, it seemed odd that it would sass me like this.