Do you want to laugh?
Get the original of giana sisters, copy with a normal copy on another disk (or today, take the g64 and convert it to D64)
Then just modifying ONE byte totally avoids the protection keeping everything else.
Indeed. I had my share of laughter back at the time. That was quite common with Rainbow Arts titles. Apparently, quite some effort went into writing all kinds of detection and penalty routines, but no effort at all actually protecting the code. Usually, all you had to do is searching for the reset jump, which they executed after self destruction. Search for the condition leading there, and then search those bytes with a disk monitor and disable that condition.
One title was even sillier. There was a loader written in BASIC, with the sole purpose of checking for the intended disk error and if found, load the real program, which was not protected on its own. All you had to do in order to disable the protection was deleting that "loader". Anyone with novice BASIC skills could figure that out.