The developer (my customer) of the Atari ST graphics program was wicked :) When he detected (in runtime at random places) that the program was cracked, he added

The developer (my customer) of the Atari ST graphics program was wicked :) When he detected (in runtime at random places) that the program was cracked, he added a invisible watermark to the output of the graphics program. When someone produced public, commercial work with a cracked version of his software, he could prove it was done with a non-paid cracked version of his software. He could "convince" several user to pay him retrospective.