I was never very advanced at cracking, but I do remember back on the C64 I encountered a number of games that had corrupt/unreadable sectors on the disk. Many copy programs would halt when they encountered these errors meaning you couldn't copy the disk. I had one program (no idea what it was called anymore) that would skip the errors and write everything else. To beat the copy protection, I would open the sector with the read error on the original disk, swap out for my mostly complete copy, and then write the disk error to that sector. I always thought it was kind of weird that I could write an unreadable error, but it seemed to work most of the time.