The C64 protection methods that drove me most up the wall were V-MAX! and Rapidlok, especially since V-MAX! existed in several different variations. Man, those were a pain. Harald Seeley (Alien Technology Group, who developed V-MAX! and WarpSpeed, and did a number of ports for companies like Cinemaware, was in my hometown of San Diego) eventually explained a lot of the tricks later on. Some good discussion here https://diskpreservation.rittwage.com/?pg=vmax and here https://diskpreservation.rittwage.com/dp.php?pg=rapidlok .