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
GXYPJ-4W67Q-6273F-QJ7FR-9MW4M
XCNF8-R99R9-R4XV8-2VQD7-FJP6M
(for some reason all 3s worked)
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
Or 11 - the last digit could be X (for ten) if they were using the modulo 11 check digit system.