I remember an icon editor (or something similar) for Windows 3.1, it was a shareware where you could enter a code to remove the nag screen. No crack was necessary, I basically managed to enter valid registration codes by just typing random numbers. In the end I had enough valid numbers that I could figure out the logic, it was something about the sums of digit groups.
This was true for the 10-digit CD keys Microsoft used for many products in the 90s: the first three digits could be almost anything, and the last seven digits had to sum to a multiple of 7, so, e.g., 111-1111111 was a valid product key (for any product that used the scheme).
I think it was StarCraft where we’d just try random keys until it would work, usually only three or four needed.