I used to crack C64 disk games. They were mostly trivial because I wrote a disk sector editor that would disassemble blocks on demand.

One that comes to mind were Ocean’s copy protection that was hacked by a load of 1 into the accumulator and a return. They had a “bool IsValidDisk()” type of routine.

After cracking one of their games I could crack others in less than 2 minutes directly on a copied floppy.