Back when I was a kid in the 80's. I cracked one of the Ultima games. I had it on my hard drive and didn't want to stick a floppy in every time I ran it.

The code decrypted itself, which confused debuggers, and then loaded a special sector from disk. It was a small sector buried in the payload of a larger sector, so the track was too big to copy with standard tools. The data in the sector was just the start address of the program. My fix was to change executable header to point to the correct start address.