Robotwar (a programming game in which you program robots in a vaguely BASIC-like language and then let them fight in an arena) had a copy-protection scheme that

Robotwar (a programming game in which you program robots in a vaguely BASIC-like language and then let them fight in an arena) had a copy-protection scheme that could be defeated by, as I vaguely recall, breaking into a DOS prompt and saving files to cassette, then transferring them to a normal DOS disk.