For StarCraft, the installer would do some math on all but the last number, then see if the result of that matched the last number to decide if the key was valid. So you could enter random numbers, then just retry while changing the last digit and guess a valid key in max 10 tries

Same for Counter-Strike 1.6, but there it was

aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

only illegal if Microsoft still supported XP.