[jasomill]

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).

[bombcar]

I think it was StarCraft where we’d just try random keys until it would work, usually only three or four needed.